Signature campaigns provide instructions for you to do within a span of a week. Usually, the goal is for you to post around 10-15 per week whilst avoiding the boards the campaign asked you not to. Scroll further down the bounty thread and you'll see signatures for a certain rank. If you want to join the bounty, copy the given signature then go to your profile>forum profile information (left side)>signatures (paste the signature)>apply for the bounty>and you're good to go.