Each round starts with the double in the middle: 12-12 in the first round, then 11-11, down to 0-0.
Play one tile on your own train or on the shared train. Can't play? Draw one; if it plays, play it, if not, put out your marker: your train is open to everyone until you play on it.
A double must be covered right away, by you or the next players, before anything else is played.
The round ends when someone plays their last tile, or nobody can play. Everyone writes down the dots left in hand (0-0 counts 0).