Maybe others signature designer using photoshop to make a image became pixel art.
But you can use such a web app to help you to create your signature design like Symbol list (full list of symbol that you can attach to your sig design) , Gradient BBcode colorizer( a tool that you can make a gradient color in hex color code) and letter count.
Also you must know the signature restrictions for every rank of this forum :

Signature restrictions

There are restrictions on signatures, based on your activity/rank.