Part 1: The control to the bottom If you know how to overhand shuffle, you know that part of the shuffle includes pulling a few cards off the top of the deck over and over again. To control a card from the top of the deck to the bottom, just make sure you peel only ONE card off the top. After that, shuffle as normal, and by the time you've run through the deck, your card will be on the bottom.
Part 2: Keeping the card at the bottom When you begin your second round of shuffling, pinch the top and bottom cards of the deck, as you pull out the brick of cards from the middle. If executed correctly, this will allow you to pull out almost all the cards of the deck, leaving only the top and bottom cards in your left hand. Most importantly, as you continue to shuffle, the bottom card will remain on the bottom of the deck. Keep repeating this part as often as you like.
Part 3: The Reveal So you've got the mark's card on the bottom of the deck. Now the trick is to reveal it in a surprising manner: Holding the cards in your hand, ask the mark to cut off as many cards as they like and place them on the table. To "mark" their cut, lay the leftover cards from your hand "cross-ways" over these cards, forming an "X". At this point, the controlled card (the one that was on the bottom of the deck) will be on the bottom of the upper pack of cards. Take a few seconds to recap how you got there (time delay is important), then turn over the top packet of cards to reveal their card. (Remember, the time delay is important to make the mark forget which pack of cards came from where).