Deck of cards, envelope, paper that says “You will choose the 6 of diamonds”
Place the paper that says “You will choose the 6 of diamonds” in an envelope and set it aside. Then place the 6 of diamonds face down on the top of the deck.
Have your assistant choose a number between 5 and 20. Demonstrate how you want your assistant to deal the cards and count out the number your assistant chose. Then place those cards back on top of the deck face down.
Now have your assistant count out the number cards and turn over the last card. Since the 6 of diamonds card was on top, it should now be the last card your assistant will lay down. Pull out the prediction to show that they match.