It's not complex. I charge an hourly rate. It's a little more if you live far from me, since I'll have to cover my cost of transportation from my place to your place, or wherever we're meeting. Basically, I just want to give you good work at a good rate.
Typically, we discuss the goal of the tutoring (e.g., getting a certain score on the SAT, admission to a certain college, obtaining an athletic scholarship). I review the student's previous test scores, homework, papers, whatever is relevant. Then we do a diagnostic test pertinent to the subject and/or review a test the student has done in advance. Then we set up a study plan with goals and steps to reach those goals.
I always buy the most recent test prep books and read the news regarding tests.