Once you reach member status you are generally able to participate in signature campaigns. The simplest way to put it, the way you become a member is to post at least 60 times, and post at least once per two week period for ~2 months.
Once you have been promoted to "member" you can find a signature campaign on the forums, sign up by meeting the criteria (usually have positive or neutral trust, and have a certain ranking), the number of posts your account has currently and your payment address.
You should read the OP of the signature campaign as it will tell you hot to get paid. Usually you will need to send a PM to the person handling the campaign with the number of posts you have posted and your BTC address.
Yes, and be aware that you have to generate content.
No "+1" or smiley faces.
So it's actually only worth it if you are really interested and would read the forum anyway.
Sure, but the signature campaigns did change the nature of spam on this forum. It used to be just spamming referral links or links to shady websites. Now we have people padding post counts.