That's done by "wrapping" the entire forum in a table.
Edit your generic header with HTML like this. (Basic example)
<table width="100%" border="0"
<tr>
<td width="10%" align="center" valign="top">
Here is your left sidebar
</td>
<td align="center" valign="top">
<!--The forum ends up going here-->
Then in your generic footer file put this:
</td>
<td width="10%" align="center" valign="top">
Put your right sidebar HTML here
</td>
</tr>
</table>
Expand on that as you will, but it's a pretty simple method to "wrap" the forum in a table, and thus give yourself sidebars. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />