So after you go to the page titled "Edit Signature," scroll down to the box where you can edit your signature, and basically all you need to do is enter your signature there and then scroll down and hit "Save Signature." Assuming you didn't break space/size constraints, it should edit the signature.
I don't know whether you planned on having a text or image in your signature, but if you plan on having an image, hit that portrait marked "Insert Image" (it's like a yellow box with some mountains in it), and enter the URL to your signature (you can get a URL if you go to
ImageShack® - Image Hosting, upload the image there, then use the last link which is something like "direct link"). There is also an option to upload it from your computer or whatever, so you can also do that.
Hopefully that should be able to help you; if you have any questions just ask. Good luck!
~Moved to Help & Suggestions