Click on a card to select it, then click a square of your garden to play it in. Green flower cards can be placed on an empty square to plant a flower. Blue water cards are played on top of flowers to water them.
When a flower has been watered enough, it will have all its petals and you will earn a few cards back into your hand.
If you can get a flower to fully bloom in every square of your garden, you'll clear the game and get a score bonus.