You just need to be Post graduate student or good at talking to students. No interview required. The process involves asking you if you are willing to help with supervising undergraduates during their lab work. The let you know that you will only be working for 6 hours a week or less. The process is really straight forward. Sometimes they just approach you without you offering to help at all because they need more helpers since postgraduate students do graduate and leave.

Written application of interest. Process fairly informal. I believe most of the "interview process" goes on behind the scenes, evaluating your academic ability. I could have demonstrated modules I had not taken, which is slightly worrying for the students but great as an applicant.