There are three section on your account profile when you can add adds which will be displayed among your all post. One is avatar field which you can see at my left side image of human resource right now and in below my post you will see human resource ad banner. I am currently in humanresource campaign and they pay me for wearing their avatar and signature. So you first need to have some decent rank like member, full member etc and then whenever there is some place is open in any of the campaign over here: https://bitcointalk.org/index.php?topic=615953.0
You can ask the campaign manager to join, if they approve you can start posting constructive post to get paid weekly or on daily basis.

NOTE: some has already explained above but i have tried to go deeper and in simpler form, hope you will understand little bit now.