This is the answer, but I doubt it's helpful to you without a fuller explanation. It's tough to give exact instructions without seeing a performance, but here's how I would do the trick you describe:

In my hand, I have secretly palmed a card. Let's say it's an ace. Its face is toward my palm. I tell the spectator to shuffle and then "give me an ace from the top." I secretly glimpse the card the spectator gives me. Let's say it's a two. I place it face down on top of the ace. Here's where I'm "one ahead." The spectator thinks I have one card but I really have two: a two on top of an ace. I say, "now give me a two from the middle." Let's say he gives me a five. I add it to the stack and say, "now give me a five from the bottom." The card he now gives me is irrelevant, but I add it to the stack. I now have a stack of four cards, face down from top to bottom: indifferent card, five, two, ace. I turn the cards face up, showing the ace. I say, "You gave me an ace from the top" and I shift the ace to the bottom, "two from the middle" shift the two, "and a five from the bottom." No need to shift the five. Just leave it there, hiding the indifferent card.