Shuffle and cut the deck. No sleights of hand are needed here.
Pretend to select a random number of cards but actually hand her 13 cards.
Ask her to pocket as many as she wants as you turn around, and with the remaining cards (non-pocketed), memorize one. Once memorized, have her place the non-pocketed cards back on top of the deck.
Turn back around and arrange the top 12 cards in the shape of a clock. Do this COUNTER-CLOCKWISE, starting at 12:00 and working backwards, 11:00, 10:00, and so on.
Ask her to remove the cards from her pocket and count them.
Explain that you can tell where her card is from the number she just said. If she said 6, as in she pocketed 6 cards, then her card is the one at the 6:00 hour.
Have her name her card as you turn over the hour value corresponding to the number of pocketed cards. It will always work.