Check
this thread, it has a compilation of all currently active signature campaigns and a brief description of each one.
You should start by adding the signature suitable for your rank before even going to the uid step.
Not necessarily, I applied without wearing the signature, maybe I was lucky (in fact, I simply joined this campaign because they accepted newbies, and it turned out to be a really nice one).
Are you talking about yobit or another compagn? Because when I joined it, it automatically check if you have the signature and the personal message or not.
Anyway, in order to be accepted in yobit, you have to simply follow the steps on their thread.
Sign up on their website, add the signature adapted with your rank and the personal message.
Find your uid then add it to your yobit account, a message will appears confirming your registration.