This one is a drag to make. Very bad documentation and even less examples. Anyway what you need to do is go to module folder of simplenews and copy from subfolder "theme" files like simplenews-newsletter-body.tpl.php, simplenews-newsletter-footer.tpl.php to your default theme folder and edit them there. And what you need to do is make most of the html. It also doesn't help that stupid Microsoft decided to use word for rendering html emails in latest outlook version so CSS is gone and you need to do whole page using tables for positioning if you want to have your newsletter to be readable by outlook users and there are lots of them, so back to 2000 or whatever year it was. Anyway
you have node object in body template and most will be build there. So I am puttin here and example of template I made.
<table width="720" border="0" align="center" cellpadding="0" cellspacing="0">
<tr>
<td height="129" align="center"><img src="http://www.arabba.hr.hr/sites/all/themes/fusion/fusion_starter/css/img/tower_zaglavlje.png" alt="Tower zaglavlje" width="700" height="129" border="0" usemap="#Map" style="display:block;" title="Tower zaglavlje"></td>
</tr>
<tr>
<td><table width="720" border="0" cellspacing="0" cellpadding="0">
<tr>
<td width="10" height="66" rowspan="3"><img width="10" height="66" border="0" style="display:block;" title="Tower traka L" alt="Tower traka L" src="http://www.arabba.hr.hr/sites/all/themes/fusion/fusion_starter/css/img/l_traka.gif"></td>
<td width="700" height="10" bgcolor="#FFFFFF"> </td>
<td width="10" height="66" rowspan="3"><img width="10" height="66" border="0" style="display:block;" title="Tower traka L" alt="Tower traka L" src="http://www.arabba.hr.hr/sites/all/themes/fusion/fusion_starter/css/img/d_traka.gif"></td>
</tr>
<tr>
<td width="700" height="40" align="right" valign="middle" bgcolor="#e22780" class="naslov">NEWSLETTER <?php print format_date($node->created, 'custom', 'F, Y' ); ?></td>
</tr>
<tr>
<td width="700" height="14" bgcolor="#FFFFFF"> </td>
</tr>
</table></td>
</tr>
<tr>
<td width="700" height="200" align="center"><img src="http://www.arabba.hr.hr/sites/all/themes/fusion/fusion_starter/css/img/tower_slika.jpg" width="700" height="200"/></td>
</tr>
<tr>
<td align="center"><table width="700" border="0" cellspacing="0" cellpadding="0" bgcolor="#FFFFFF">
<tr bgcolor="#FFFFFF">
<td width="700" height="20"> </td>
</tr>
<tr>
<td bgcolor="#FFFFFF"><?php print $body; ?></td>
</tr>
<tr>
<td bgcolor="#FFFFFF"> </td>
</tr>
</table></td>
</tr>
<tr>
<td width="700" height="20"><table width="720" border="0" cellspacing="0" cellpadding="0">
<tr>
<td width="10" height="115" rowspan="2"><img width="10" height="115" border="0" style="display:block;" title="Tower traka DL" alt="Tower traka DL" src="http://www.arabba.hr.hr/sites/all/themes/fusion/fusion_starter/css/img/d_l_traka.gif" /></td>
<td width="700" bgcolor="#FFFFFF"> </td>
<td width="10" height="115" rowspan="2"><img width="10" height="115" border="0" style="display:block;" title="Tower traka DD" alt="Tower traka DD" src="http://www.arabba.hr.hr/sites/all/themes/fusion/fusion_starter/css/img/d_d_traka.gif" /></td>
</tr>
<tr>
<td width="700" class="downer" align="center" valign="middle" bgcolor="#e22780"><a class="logo" href="http://www.arabba.hr.hr"><img src="http://www.arabba.hr.hr/sites/all/themes/fusion/fusion_starter/css/img/tower-newsletter_bt_logo.gif" alt="" title="" class="" width="360" height="57" /></a></td>
</tr>
</table>
</td>
</tr>
</table>
<map name="Map" id="Map">
<area shape="rect" coords="622,94,678,113" href="http://www.arabba.hr.hr/""" />
</map>Also note that you need to upload mail.css file to your theme ROOT folder (subtheme folder, Check attached file for example). You also want your CSS to be added inline in code and this would work only if you enabled Mime Mail CSS Compressor module that comes with MIME module, as plain attaching CSS file to newsletter would also not work in some email clients.