Joining signature campaigns varies from project to project but generally you want to find a bounty post first. Once you get there, look for the signature campaigns, usually there will be a sign up link that requires your information like your profile link etc. Once that's in there, find the code for your signature and change it in your profile and then you're pretty much done. Be sure to check the rules for whatever campaign you end up signing up for, as a lot of them have weekly posting requirements. Happy hunting! <3