The plugin allows you to customize the newsletter template. Normally there is an header and an html footer and you can put your html code.
The best way to do templates is to create an html table.
The table should be include the logo image on top and maybe some statics links. Also the header must end with an open <td> tag that should be the first closing tag in the footer.
So your header will end with <td> and the footer should start with the same closing tag </td>. In this way the newsletter content will be placed in the <td></td>
Remember: Every list has his own Template (header and footer code), to manage the template you have to enter in List management and setup the sender address and the template as show in the following png:
Template Options page
