You show a blank card, while spreading out the deck to display the rest of the cards as being regular. They all have faces except the original blank card.
The spectator has a completely free choice to pick and sign a card of their choosing.
They then are allowed to examine both the blank card AND the empty card box.
The magician then shuffles the deck and clearly puts the signed, chosen card back in the middle before shuffling some more.
Then the blank card is placed in the middle of the deck, protruding for half it's length. The deck is then placed in the card box to further rule out any tampering from the magician, while the blank card is still plainly in view. Now comes the magic...The magician acts like he is going to push the card all the way into the deck, but about halfway through, he turns it around to show you that the blank card has changed to the chosen card in the blink of an eye!