Rules are simple, player 1 posts an If, player 2 responds a Then and a new If.
Example:
Player 1: If Zombies
Player 2: Then Break Glass
If Sparkling Vampires
Player 1: Then break eyes
If Andrew Jackson were to be elected
Needless to say, the If doesn't have to be related to your Then, but it can be.
Start:
If orange bandicoots