You will have to learn a display move. Hold two cards in one hand. Naturally, to spectators, you'll be displaying what look to be two cards, one face up and the other face down.
Turn over your hand. As you turn your hand, use your fingers to shift the positions of the cards. With your hand turned over, it will look to spectators as if you simply turned your hand over to display the face of the second card. In reality, the card faces are part of the same card.
