I always just tell the spectator, "...Now place the card in the middle of the pack." as I cut the cards to the middle, extending my index finger, to casually point at the center of the bottom portion of the pack, (which is face down, of course). Meanwhile, by the action of rotating my wrist to point at the bottom portion of the deck (maybe giving it a bit of a quick "tap" even), I can catch a momentary glimpse of the bottom card of the
top half of the deck of cards, which has it's face towards me, right in front of me; can't miss it. I simply memorize
that card as the spectator does what I say and places their card on top of the bottom half of the deck of cards. Once I place the top half on top of the bottom half, I now know that the card I have memorized will be
directly to the right of the spectator's card, when I fan the cards in my left hand from
left to right. But BEFORE I do that, I give the deck of cards a bunch of real cuts as I make small talk with the spectator. This reinforces in their minds that the deck of cards
really is getting all kinds of mixed up. In truth though, the order of the cards doesn't really change; if the card
I memorized ends up on the bottom of the deck, then the spectators card is now
on top of the deck. (I love it when that happens!)
--Muolidae the Magician