Don't recall that one being very difficult, just get in all the humiliation you can because of the cat ears. Picture/search/paddle/rub/spank with her straddling (as HM)/use dildo on desk.
If you mean how do you start a sequence, you should get a message when you advance enough other stories to unlock PET part 2. You need to do a body search on Alice with Rachel following you, then you need to find Debbie protecting Rachel from Donna teasing about her cat ears while on patrol. That unlocks a scene where you let Debbie play with Rachel in your office. After that, buy dildos and take Rachel through the training. Her next punishment after you finish training will be from 7 to 8.