You need to prepare your marked card.

For this you will need something called "Security UV PEN."

Anything you write with this pen is visible only under UV light (black light).

So you will need also source of black light.

Step by step

  1. Force marked card to spectator. Use your favorite method - one is described in video below
  2. Let the spectator to shuffle the deck
  3. Spread the cards and if you have possibility then also turn the lights off
  4. Inspect the cards by black light - it will be visible that one card is different from others
  5. Show to spectator that the card that was chosen by him is the only marked card in the deck