If you take and pass two subject tests, they will contact you and do an online interview in which you need to give them a sample of tutoring (the interviewer will play the part of the online student). You need to prepare for this by watching several training videos, and by downloading the classroom software.
If you pass the interview, they will do a background check on you that can take several weeks. If you pass, you can start logging in to the classroom as "Available," and they will start sending students your way.