Click on a banner you like, it will usually take you to the site where it was created.
Do whatever that particular one needs you to do then there is usually a button like 'generate banner' or some such thing.
You will usually be presented with two pieces of code, HTML and BB Code. Copy the BB Code
Come back here and look at the top of the page, you will see a button called Settings.
In settings you will see, in teh left sidebar, an option to Edit your Signature.
Paste in your BB code and save.
That's it, you're done!