Start with the 3 of spades set up as the 10th card down from the top. No matter which number they pick from 10-20, this is the card they'll end up with. To prep the prediction, first use a regular pen to write "you will pick the 3 of ." Then, use one of the pilot Frixion pens to draw over the 3 of with the 8 of , and you're set!