This is pretty straightforward, so I'm not going to explain it in depth. Here's the high points:

1. 1. Ensure you have only one signature setup in Mail. If you have one already, make a new one, and make a note of the time, because you'll need that to discern the signature to be modified from those which are not.
2. 2 .Quit Mail
3. 3. Create your signature in an HTML editor, or script it manually if you can.
4. 4. Save the file to your hard drive, and open the page in Safari
5. 5. Click File, Save As..., and save the page as a web archive.
6. 6. Go to Part 2