Basically 3 step process:
1) enter your info on the website
2) they call you and do a phone screening interview
3) you install some videoconferencing software (easy), then they do a more detailed interview using this, which could last up to 1 hour. The last interview seems to be basically intended to confirm that you are qualified in the subjects you claim to be able to tutor, so they are specific academic questions. For example if you want to tutor algebra, they will ask you algebra questions, etc. They might also ask you about experience etc., but in my case that aspect was mostly covered in the phone interview.
You are also supposed to be prepared to teach a short lesson on some topic. I expected to be able to choose the topic, so I prepared a chemistry lesson, but in the actual interview the interviewer chose the topic and I had to extemporaneously come up with a lesson, so be prepared for this. The topic she chose was physics (wave equation) which is not one of my stronger subjects, but fortunately I did OK. It is probably good to be conservative in the topics that you list as being able to tutor because if you list ones where you are not 100% confident, they might ask you to teach a topic you don't know well and then you will not be able to pass the interview. On the other hand, the more subjects you can teach, presumably the more assignments you will get. I recommend explaining the answers to the questions as you work them, as if you were with a student, because this is the main skill you are trying to demonstrate - i.e. not only state what the answer is but try to explain clearly why it is correct and what is the logical process used to find the answer. (you might run into time limitations though)
Another thing, make sure you have a gmail account and it is working. Because they might send you questions with google docs and if you have trouble logging in to gmail for any reason, this might cause a glitch. In my case I had not previously logged in to my google account from that IP address so google went through the security check thing, and the security code it wanted to send was to my previous cell phone which was in another country!
For math it would also be a very good idea to have a graphing calculator ready to go, I felt the need for one during the interview, though in the end I passed it without one.