There are 21 total toilets around South Park that you must sit on and play a mini game. The toilets each have their own difficulty ranging from 1-star to 4-star and in order for a toilet to count towards your mastery level you must fill up the bar to the top star.
I found toilets ranging from 1-3 stars were fairly easy but could not master any of the 4-star toilets. If you are also struggling with any of the toilets or you just want to get them done quickly then go to options menu and turn on the mini game assist. This makes it so you simply just need to hold R1 during the toilet mini games and you'll see the bar fill itself up to the top without needing to follow any of the commands on the screen.