https://stripo.email would a be great place to start for a drag and drop editor to customize your design, here is something i created quickly which gives you a header / body / footer
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:o="urn:schemas-microsoft-com:office:office" style="font-family:arial, 'helvetica neue', helvetica, sans-serif">
<head>
<meta charset="UTF-8">
<meta content="width=device-width, initial-scale=1" name="viewport">
<meta name="x-apple-disable-message-reformatting">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta content="telephone=no" name="format-detection">
<title>DRW</title><!--[if (mso 16)]>
<style type="text/css">
a {text-decoration: none;}
</style>
<![endif]--><!--[if gte mso 9]><style>sup { font-size: 100% !important; }</style><![endif]--><!--[if gte mso 9]>
<xml>
<o:OfficeDocumentSettings>
<o:AllowPNG></o:AllowPNG>
<o:PixelsPerInch>96</o:PixelsPerInch>
</o:OfficeDocumentSettings>
</xml>
<![endif]--><!--[if !mso]><!-- -->
<link href="https://fonts.googleapis.com/css?family=Open+Sans:400,400i,700,700i" rel="stylesheet"><!--<![endif]-->
<style type="text/css">
#outlook a {
padding:0;
}
.es-button {
mso-style-priority:100!important;
text-decoration:none!important;
}
a[x-apple-data-detectors] {
color:inherit!important;
text-decoration:none!important;
font-size:inherit!important;
font-family:inherit!important;
font-weight:inherit!important;
line-height:inherit!important;
}
.es-desk-hidden {
display:none;
float:left;
overflow:hidden;
width:0;
max-height:0;
line-height:0;
mso-hide:all;
}
[data-ogsb] .es-button {
border-width:0!important;
padding:10px 20px 10px 20px!important;
}
.es-button-border:hover a.es-button, .es-button-border:hover button.es-button {
background:#56d66b!important;
border-color:#56d66b!important;
}
.es-button-border:hover {
border-color:#42d159 #42d159 #42d159 #42d159!important;
background:#56d66b!important;
}
@media only screen and (max-width:600px) {p, ul li, ol li, a { line-height:150%!important } h1, h2, h3, h1 a, h2 a, h3 a { line-height:120% } h1 { font-size:30px!important; text-align:left } h2 { font-size:24px!important; text-align:left } h3 { font-size:20px!important; text-align:left } .es-header-body h1 a, .es-content-body h1 a, .es-footer-body h1 a { font-size:30px!important; text-align:left } .es-header-body h2 a, .es-content-body h2 a, .es-footer-body h2 a { font-size:24px!important; text-align:left } .es-header-body h3 a, .es-content-body h3 a, .es-footer-body h3 a { font-size:20px!important; text-align:left } .es-menu td a { font-size:14px!important } .es-header-body p, .es-header-body ul li, .es-header-body ol li, .es-header-body a { font-size:14px!important } .es-content-body p, .es-content-body ul li, .es-content-body ol li, .es-content-body a { font-size:14px!important } .es-footer-body p, .es-footer-body ul li, .es-footer-body ol li, .es-footer-body a { font-size:14px!important } .es-infoblock p, .es-infoblock ul li, .es-infoblock ol li, .es-infoblock a { font-size:12px!important } *[class="gmail-fix"] { display:none!important } .es-m-txt-c, .es-m-txt-c h1, .es-m-txt-c h2, .es-m-txt-c h3 { text-align:center!important } .es-m-txt-r, .es-m-txt-r h1, .es-m-txt-r h2, .es-m-txt-r h3 { text-align:right!important } .es-m-txt-l, .es-m-txt-l h1, .es-m-txt-l h2, .es-m-txt-l h3 { text-align:left!important } .es-m-txt-r img, .es-m-txt-c img, .es-m-txt-l img { display:inline!important } .es-button-border { display:inline-block!important } a.es-button, button.es-button { font-size:18px!important; display:inline-block!important } .es-adaptive table, .es-left, .es-right { width:100%!important } .es-content table, .es-header table, .es-footer table, .es-content, .es-footer, .es-header { width:100%!important; max-width:600px!important } .es-adapt-td { display:block!important; width:100%!important } .adapt-img { width:100%!important; height:auto!important } .es-m-p0 { padding:0!important } .es-m-p0r { padding-right:0!important } .es-m-p0l { padding-left:0!important } .es-m-p0t { padding-top:0!important } .es-m-p0b { padding-bottom:0!important } .es-m-p20b { padding-bottom:20px!important } .es-mobile-hidden, .es-hidden { display:none!important } tr.es-desk-hidden, td.es-desk-hidden, table.es-desk-hidden { width:auto!important; overflow:visible!important; float:none!important; max-height:inherit!important; line-height:inherit!important } tr.es-desk-hidden { display:table-row!important } table.es-desk-hidden { display:table!important } td.es-desk-menu-hidden { display:table-cell!important } .es-menu td { width:1%!important } table.es-table-not-adapt, .esd-block-html table { width:auto!important } table.es-social { display:inline-block!important } table.es-social td { display:inline-block!important } .es-desk-hidden { display:table-row!important; width:auto!important; overflow:visible!important; max-height:inherit!important } .es-m-p5 { padding:5px!important } .es-m-p5t { padding-top:5px!important } .es-m-p5b { padding-bottom:5px!important } .es-m-p5r { padding-right:5px!important } .es-m-p5l { padding-left:5px!important } .es-m-p10 { padding:10px!important } .es-m-p10t { padding-top:10px!important } .es-m-p10b { padding-bottom:10px!important } .es-m-p10r { padding-right:10px!important } .es-m-p10l { padding-left:10px!important } .es-m-p15 { padding:15px!important } .es-m-p15t { padding-top:15px!important } .es-m-p15b { padding-bottom:15px!important } .es-m-p15r { padding-right:15px!important } .es-m-p15l { padding-left:15px!important } .es-m-p20 { padding:20px!important } .es-m-p20t { padding-top:20px!important } .es-m-p20r { padding-right:20px!important } .es-m-p20l { padding-left:20px!important } .es-m-p25 { padding:25px!important } .es-m-p25t { padding-top:25px!important } .es-m-p25b { padding-bottom:25px!important } .es-m-p25r { padding-right:25px!important } .es-m-p25l { padding-left:25px!important } .es-m-p30 { padding:30px!important } .es-m-p30t { padding-top:30px!important } .es-m-p30b { padding-bottom:30px!important } .es-m-p30r { padding-right:30px!important } .es-m-p30l { padding-left:30px!important } .es-m-p35 { padding:35px!important } .es-m-p35t { padding-top:35px!important } .es-m-p35b { padding-bottom:35px!important } .es-m-p35r { padding-right:35px!important } .es-m-p35l { padding-left:35px!important } .es-m-p40 { padding:40px!important } .es-m-p40t { padding-top:40px!important } .es-m-p40b { padding-bottom:40px!important } .es-m-p40r { padding-right:40px!important } .es-m-p40l { padding-left:40px!important } }
</style>
</head>
<body style="width:100%;font-family:arial, 'helvetica neue', helvetica, sans-serif;-webkit-text-size-adjust:100%;-ms-text-size-adjust:100%;padding:0;Margin:0">
<div class="es-wrapper-color" style="background-color:#666666"><!--[if gte mso 9]>
<v:background xmlns:v="urn:schemas-microsoft-com:vml" fill="t">
<v:fill type="tile" color="#666666" origin="0.5, 0" position="0.5, 0"></v:fill>
</v:background>
<![endif]-->
<table class="es-wrapper" width="100%" cellspacing="0" cellpadding="0" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;padding:0;Margin:0;width:100%;height:100%;background-repeat:repeat;background-position:center top;background-color:#666666">
<tr>
<td valign="top" style="padding:0;Margin:0">
<table class="es-header" cellspacing="0" cellpadding="0" align="center" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;table-layout:fixed !important;width:100%;background-color:transparent;background-repeat:repeat;background-position:center top">
<tr>
<td align="center" style="padding:0;Margin:0">
<table class="es-header-body" cellspacing="0" cellpadding="0" bgcolor="#ffffff" align="center" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;background-color:#FFFFFF;width:720px">
<tr>
<td style="padding:0;Margin:0;padding-top:35px;padding-bottom:35px;background-color:#efefef" bgcolor="#efefef" align="left">
<table width="100%" cellspacing="0" cellpadding="0" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td valign="top" align="center" style="padding:0;Margin:0;width:720px">
<table width="100%" cellspacing="0" cellpadding="0" role="presentation" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td style="padding:0;Margin:0;font-size:0px" align="center"><img src="https://invoicing.co/images/new_logo.png" alt style="display:block;border:0;outline:none;text-decoration:none;-ms-interpolation-mode:bicubic" width="167"></td>
</tr>
</table></td>
</tr>
</table></td>
</tr>
</table></td>
</tr>
</table>
<table class="es-header" cellspacing="0" cellpadding="0" align="center" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;table-layout:fixed !important;width:100%;background-color:transparent;background-repeat:repeat;background-position:center top">
<tr>
<td align="center" style="padding:0;Margin:0">
<table class="es-header-body" cellspacing="0" cellpadding="0" bgcolor="#ffffff" align="center" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;background-color:#FFFFFF;width:720px">
<tr>
<td align="left" style="padding:0;Margin:0;padding-top:20px;padding-bottom:20px">
<table width="100%" cellspacing="0" cellpadding="0" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td valign="top" align="center" style="padding:0;Margin:0;width:720px">
<table width="100%" cellspacing="0" cellpadding="0" role="presentation" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td align="left" style="padding:0;Margin:0;padding-left:40px;padding-right:40px"><p style="Margin:0;-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;font-family:'open sans', 'helvetica neue', helvetica, arial, sans-serif;line-height:21px;color:#333333;font-size:14px">$body</p></td>
</tr>
</table></td>
</tr>
</table></td>
</tr>
</table></td>
</tr>
</table>
<table class="es-footer" cellspacing="0" cellpadding="0" align="center" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;table-layout:fixed !important;width:100%;background-color:transparent;background-repeat:repeat;background-position:center top">
<tr>
<td align="center" style="padding:0;Margin:0">
<table class="es-footer-body" cellspacing="0" cellpadding="0" bgcolor="#ffffff" align="center" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;background-color:#FFFFFF;width:720px">
<tr>
<td style="padding:0;Margin:0;padding-bottom:10px;padding-top:20px;background-color:#efefef" bgcolor="#efefef" align="left"><!--[if mso]><table style="width:720px" cellpadding="0" cellspacing="0"><tr><td style="width:243px" valign="top"><![endif]-->
<table class="es-left" cellspacing="0" cellpadding="0" align="left" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;float:left">
<tr>
<td class="es-m-p20b" align="left" style="padding:0;Margin:0;width:223px">
<table width="100%" cellspacing="0" cellpadding="0" role="presentation" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td style="padding:0;Margin:0;padding-right:10px;padding-left:40px;width:30px;font-size:0px" valign="top" align="right"><a href="https://invoicing.co" target="_blank" style="-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;text-decoration:underline;color:#FFFFFF;font-size:14px"><img src="https://fhfhm.stripocdn.email/content/guids/CABINET_e972502513414b0c0dde88d2abdc965b/images/home.png" alt style="display:block;border:0;outline:none;text-decoration:none;-ms-interpolation-mode:bicubic" width="35"></a></td>
<td align="left" style="padding:0;Margin:0">
<table width="100%" cellspacing="0" cellpadding="0" role="presentation" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td align="left" style="padding:0;Margin:0;padding-top:5px"><p style="Margin:0;-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;font-family:'open sans', 'helvetica neue', helvetica, arial, sans-serif;line-height:21px;color:#333333;font-size:14px"><a target="_blank" href="https://invoicing.co" style="-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;text-decoration:underline;color:#cc0000;font-size:18px">Visit Us</a></p></td>
</tr>
</table></td>
</tr>
</table></td>
<td class="es-hidden" style="padding:0;Margin:0;width:20px"></td>
</tr>
</table><!--[if mso]></td><td style="width:237px" valign="top"><![endif]-->
<table class="es-left" cellspacing="0" cellpadding="0" align="left" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;float:left">
<tr>
<td class="es-m-p20b" align="left" style="padding:0;Margin:0;width:237px">
<table width="100%" cellspacing="0" cellpadding="0" role="presentation" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td style="padding:0;Margin:0;padding-right:10px;padding-left:40px;width:30px;font-size:0px" valign="top" align="right"><a href="mailto:contact@invoiceninja.com" target="_blank" style="-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;text-decoration:underline;color:#FFFFFF;font-size:14px"><img src="https://fhfhm.stripocdn.email/content/guids/CABINET_e972502513414b0c0dde88d2abdc965b/images/email.png" alt style="display:block;border:0;outline:none;text-decoration:none;-ms-interpolation-mode:bicubic" width="35"></a></td>
<td align="left" style="padding:0;Margin:0">
<table width="100%" cellspacing="0" cellpadding="0" role="presentation" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td align="left" style="padding:0;Margin:0;padding-top:5px"><p style="Margin:0;-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;font-family:'open sans', 'helvetica neue', helvetica, arial, sans-serif;line-height:21px;color:#333333;font-size:14px"><a target="_blank" href="mailto:contact@invoiceninja.com" style="-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;text-decoration:underline;color:#cc0000;font-size:18px">Email Us</a></p></td>
</tr>
</table></td>
</tr>
</table></td>
</tr>
</table><!--[if mso]></td><td style="width:20px"></td><td style="width:220px" valign="top"><![endif]-->
<table class="es-right" cellspacing="0" cellpadding="0" align="right" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;float:right">
<tr>
<td align="left" style="padding:0;Margin:0;width:220px">
<table width="100%" cellspacing="0" cellpadding="0" role="presentation" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td style="padding:0;Margin:0;padding-right:10px;padding-left:40px;width:30px;font-size:0px" valign="top" align="right"><a href="tel:0220452299" target="_blank" style="-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;text-decoration:underline;color:#FFFFFF;font-size:14px"><img src="https://fhfhm.stripocdn.email/content/guids/CABINET_e972502513414b0c0dde88d2abdc965b/images/phone.png" alt style="display:block;border:0;outline:none;text-decoration:none;-ms-interpolation-mode:bicubic" width="35"></a></td>
<td align="left" style="padding:0;Margin:0">
<table width="100%" cellspacing="0" cellpadding="0" role="presentation" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td align="left" style="padding:0;Margin:0;padding-top:5px"><p style="Margin:0;-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;font-family:'open sans', 'helvetica neue', helvetica, arial, sans-serif;line-height:21px;color:#333333;font-size:14px"><a target="_blank" href="tel:555-123-4567" style="-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;text-decoration:underline;color:#cc0000;font-size:18px">Call Us</a></p></td>
</tr>
</table></td>
</tr>
</table></td>
</tr>
</table><!--[if mso]></td></tr></table><![endif]--></td>
</tr>
</table></td>
</tr>
</table>
<table class="es-footer" cellspacing="0" cellpadding="0" align="center" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;table-layout:fixed !important;width:100%;background-color:transparent;background-repeat:repeat;background-position:center top">
<tr>
<td align="center" style="padding:0;Margin:0">
<table class="es-footer-body" cellspacing="0" cellpadding="0" bgcolor="#ffffff" align="center" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px;background-color:#FFFFFF;width:720px">
<tr>
<td style="padding:0;Margin:0;padding-top:20px;padding-bottom:20px;background-color:#333333" bgcolor="#333333" align="left">
<table width="100%" cellspacing="0" cellpadding="0" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td valign="top" align="center" style="padding:0;Margin:0;width:720px">
<table width="100%" cellspacing="0" cellpadding="0" role="presentation" style="mso-table-lspace:0pt;mso-table-rspace:0pt;border-collapse:collapse;border-spacing:0px">
<tr>
<td align="center" style="padding:0;Margin:0"><p style="Margin:0;-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;font-family:'open sans', 'helvetica neue', helvetica, arial, sans-serif;line-height:21px;color:#ffffff;font-size:14px"> © 2022 <a target="_blank" style="-webkit-text-size-adjust:none;-ms-text-size-adjust:none;mso-line-height-rule:exactly;text-decoration:none;color:#FFFFFF;font-size:14px" href="https://invoicing.co">Invoice Ninja</a>. All Rights Reserved</p></td>
</tr>
</table></td>
</tr>
</table></td>
</tr>
</table></td>
</tr>
</table></td>
</tr>
</table>
</div>
</body>
</html>