First of all, you'll need to get software such as Photoshop or Gimp to actually make the signature.
Usually you'll start off with a background of your choice, along with renders/images you want to use, and maybe text as well.
Then you can look in the GFX section for tutorials on effects or styles that you might want to use.
The maximum dimensions for a signature is 600x300, if I recall correctly.
If you're ever in need, you can always make a signature request in the GFX section.