So easy to do! As a spectator to think of any card from a deck of 52 playing cards. Then show them your computer, perspex cards. Tell them that the cards are magical computers that can analyse their brain to perform real magic.
Show the cards one by one to the spectator. Always make sure you ask them if they can see their card. If they see their card on one of the perspex sheets then that card stays in your hands. If they do not see it then put that perspex sheet away in your back pocket.
You will then be left with a handful of cards and from here you can use the cards to name the thought of card. Square the cards up that are left in your hands and you will notice now that there is a gap in one of the card patterns.
Try this at home first. Think of a card and then with the perspex sheets which contain your thought of card - put them to one side. Align up the remainder and witness how easy the reveal is. There will be a gap to reveal the though of card on the sheets of perspex.
Perform this trick well and make it believable!