Here's how:
- Click on the link, "User CP" at the top left of any page.
- Scroll down the menu on the left side of the CP page.
- Under Settings and Options, click on "Edit Signature."
- That will take you to a page with a box that looks just like the regular "reply" box -- enter whatever you want for your signature, then click "Save Signature."
It's also not a bad idea to preview it.