You first need to design in BBS code, which is what this forum uses for signature code effects, colors, etc.
Here are some resources for you:
Another tip, you can see the text used to create different signature designs by looking at the signature codes different signature campaigns offer for their participants. You can learn by reading this code (once you know what BBS code is). So go to the various signature campaign threads to see what the code is to create different designs and then start building your own!
Once you're good at it you can get paid for this activity.