To start playing select the cards you want to keep and then click "Change Cards"
If you get a winning card you will be asked to guess whether the next card in the deck is red or black
Each correct guess will double your winnings. If you guess 3 cards in a row you win the current hand. A wrong guess will cause you to lose the curent hand.
Don't worry there's no real money involved... If you ever run out of credits, just refresh the page to start over.