Read and learn the beginner's version of this featured in my Beginner's Ambitious Card tutorial on this website.
Perform steps #1 to #4 from the beginner's tutorial. This tutorial begins after step #4 on the other. Right now, her chosen card should be second from the top even though she thinks it is in the middle.
Double-lift the top two cards to reveal hers. Holding it down, have her write something on her card.
Slightly bend the two cards (which she thinks is one) at the middle to cause a crease. Explain that since the card is bent, you clearly cannot do any trickery.
Place the bent cards back on top of the deck. Remove the top one, which is not hers even thought she swears it is, and place it into the middle. Be sure to tightly press down on both sides of the new top card, which is actually her bent card, so as to show it is not bent.
Tell her to snap her fingers. Right now, she "knows" her card is in the middle because the bent card was placed there.
When she snaps, release the pressure you are applying on the top card and it will prop up into bent position. It will appear as if a bent card just transported from middle to top.