The rules of Rules! are very simple: you simply follow the rules.
That might be to only tap green cards, or tap cards in descending order, or only animal cards. Whatever the rules, if you follow them then you remove cards from play, can keep playing, and are then given more rules to follow. If you don't, you run out of time and it's game over.
Sounds boring, but it's actually lots of fun.
Here's video proof.