Simple sht, just make a letter combination, and the poster beneath you will make up what it is. That same said secondary poster will also post a letter combination, and this way the little game will repeat itself until it's untimely death at in the depths of the Forum Games.
Billy: IHOP
Susie: I
Hate
Old
People
FAPT
In this scenario, Susie's next acronym is to be solved by you.
It was: FAPT
Good Luck!