Be the first to finish your cards. Play a card that matches the shape or the number on top of the pile. If you cannot play, go to market and pick one.
If the market finishes, everyone counts the cards in their hand. Lowest count wins. Stars count double.
Some people play that a 2 can block a 2 and a 5 can block a 5, and the pick passes on and adds up. Turn that on in Settings if that is how you play.