how do you make and post a banner in your signature
First you need to create the image, and then upload it. After that, you just go to your user control pannel (which you can access by clicking 'private messages' at the top-right next to your avatar), and select
edit signature. Insert the image there as you would in any post, with the URL of the place it was uploaded between tags. You can also click the mountain icon to assist you. Once you've made the desired changes, save them, and if it doesn't show up right first try refreshing.
The hardest part, I suspect, will be making and uploading an image. What you should do if you don't have experience in this area is to ask in appropriate threads in the [url=http://www.smashboards.com/forumdisplay.php?f=15]Artwork Emporium[/url] forum, such as asking for a sig in any of the sig/banner shops, specifying what general things you would like to see in it (your name, images of _____, bg color, darkness, etc). If they post an image, right-click on it, and under properties copy-paste its URL, and use that in your signature (between [IMG] tags).