To start off with, you need to have an activity to do while the challenge happens (poker, running, something....I prefer a poker game because it forces everyone to be near each other and bathrooms nearby) Everyone takes a laxative, wears an adult diaper and eats taco bell, and sits around playing poker. There are enough bathrooms for all but one person. The loser is the person that poops first, and as soon as they do everyone else can go relieve themselves. Afterwards, the loser can't change out of their clothes or diaper and has to either go somewhere public with it on or leave it on the rest of the day/through a party, etc.
Have an iPhone? Check out all 200+ new features coming in iOS 13.