It's a simple matter of copy/pasting.
1. Go to the guild's home page and choose which banner you want to use.
The original banner
[url=http://tinyurl.com/brvw9][IMG]http://tinypic.com/keyc6o.gif[/IMG][/url]
The non-swearing banner
[url=http://tinyurl.com/brvw9][IMG]http://img217.imageshack.us/img217/3822/3gbanner8jd.gif[/IMG][/URL]
2. Highlight it by holding down the left mouse button and dragging the cursor over the text.
3. Hold down the Ctrl key and press C. This will copy what you've highlighted.
4. Click on My Gaia near your avatar in the upper right corner of the window.
5. Click on Account.
6. Click on Signature just above Old School Profile.
7. Click inside the resulting box. You should see the cursor flashing. Hold down the Ctrl key and press V. This will paste the code in.
8. Click Preview to see what it will look like, or click Submit to finalize it.
9. Enjoy the banner!