Crappin' Forte: Achievement PSN of South Park: The Fractured But Whole

See: South Park The Fractured But Whole Toilet Locations (Text Guide)

There are 21 Toilets. You can get all of them after the story in free roam. None of them are missable. It's better to do them after the story because you need certain abilities to access everything (which are unlocked automatically in the endgame).
To master a toilet you must complete a button-pressing minigame and get all stars. When you've done it correctly, a green checkmark will show on the toilet. Do this to all 21 of them.