Before the trick make up a packet containing 20 black cards and one red (we'll say the Queen of Hearts.) Put the red card 10th from the top. Place this packet on top of the rest of the pack.
In performing the trick say to the audience you will remove a random number of cards from the top of the pack, but remove the top 21 cards placed there earlier and disregard the rest of the pack (you no longer require them).
Give the packet of 21 cards to the spectator and turn your back. Tell the spectator to remove between one and ten cards from the top of the packet, count the number to themselves and put these cards into their pocket.
You tell the spectator you will predict which card they will choose and write it on a piece of paper. Write down the Queen of Hearts (or whatever the red card was), fold the paper and give it to a member of the audience to hold.
Turn back to face the spectator. Deal out from right to left ten cards from the remainder of the packet. Ask the spectator how many cards they have in their pocket and remind them that they had a free choice of between one and ten. Count from left to right the number they tell you. The next card to the right is theirs (it's the Queen.) Open the piece of paper and show your prediction to be correct.
To top off the trick ask them to return the cards from their pocket and tell them that not only did they pick the card on the slip of paper, but they chose the only red card in the pack. Show that all the other cards were black.