There are quite a few different banners -- are you asking specifically how to get one, or just how to add it to your signature line?
To add it to your signature line, go into "Settings" (menu at the top of the page) and choose "Edit Signature" -- if you have a banner from elsewhere -- to get the generic one offered here, go up to "Forum Actions", "Generate SigBanner" and take it from there.
Andria