Watch this video and more on Master Magic Tricks
While dribbling the deck on the table, randomly toss the spectator's card within. The card ends up on top of the deck.
Add the spectator's card to the center of the deck. Square up and their card goes right back on top.
The spectator's card elevates from the middle of the deck to the top.
The spectator's card is placed into the deck, and switches to become the card on the top of the deck.