To get you started, find the script that does the mailing and look for some lines that start with 'print'. You should see a line printing a header of some sort. After that, the stuff printed from the script is basically sent verbatim to the browser. So if you do a 'view source' on the final page, you should be able to match that up with the stuff in the print statements.

If you can match these things up, you can start to modify the script. The simplest thing to do would be to put the HTML from the rest of your site pages into the script. For example, you might want to put in print statements to print out the HTML for your site header and your footer.

As the other posters said, maintaining print statements like this quickly becomes tiresome. If you find you need to do this often, look into some templating options so you can pull the HTML out of the script itself.

Good luck!