Shanghai is a really fun game mode and the rules are fairly simple to learn.
Each player starts by trying to hit one number per round, starting on 1 and ending on 5. Hitting a double and treble increases your score by that amount respectively.
If you manage to hit a single, double and treble in the same round, your round total is multiplied by 3. This is called a Shanghai and should be your goal in each round.
The winner is the player who has the highest score at the end of 5 rounds.