Hold the card in one hand. Place your index finger and little finger on either side of the front of the card. Bend your index finger and ring finger so the tips of those fingers are pressing against the back of the card. Hold your hand flat, with the front of your hand toward the audience and the back of your hand hidden from the audience.
Straighten your middle finger and ring finger so the card flips around to the back of your hand. The top corner edges of the card should stay between your fingers so the card doesn't fall. As you do this, wave your hand up and down so the audience doesn't see the edges of the corners.
To make the card reappear, release one corner of the card from between your ring finger and little finger. Keep hold of the corner of the card that's between your index finger and middle finger. The card will flip up and be visible above your hand.