Get the pictures from the forums or in-game if you can and follow
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if you want something quick and easy. (Thanks to
@GhostPhil). You can use it for video gifs too so it'll cut and convert for you.
The not so quick and easy versions require a variety of different programs depending on what you want in your fan signature. Could be Photoshop, could be Premiere or PowerDirector.
What are you looking for in a fan signature?