start with the bottom card upside down
Don't let the spectators see that.
tell them to pick a card. don't let them see the bottom card.
This is their card
This is where it gets tricky. When their not looking at the deck flip the deck.
Tell them to put the card in the deck. If you did this right then it should have been put in upside down.
When the spectator is not looking flip the cards over to their original side.
Spread the cards out and you should see the spectators card.
Practice this trick at least 20 times before you show it to anybody.