Nice and Easy
Shuffle deck. Let someone choose any card, memorise it and place on TOP of deck. Behind your back, flip THAT first card over and bring out the whole deck. Hold deck upright, so they see bottom card - don't let them see theirs. With the bottom of the deck showing, ask if that's their card, to which they say no. At the same time you can see what THEIR card is, because you've flipped it over and it's now facing you! Put deck behind your back (be careful they don't see.) Flip their card over again and put it anywhere in deck. Bring out the deck, shuffle it or get them to shuffle, and now you can find their card!