Secure one end of the string behind your ear with tape.
Conceal the string in your hair so the unsecured end hangs free in front of your face.
Stick a pea-sized portion of magician's wax to the unsecured end of the string. If the wax isn't sticking easily, roll it between your fingers until it warms and feels soft, then mold it around the string.
Stick the wax to your thumbnail to keep it from attracting attention.
Select a card, and allow an audience member to examine it.
Reclaim the card. In doing so, surreptitiously transfer the magician's wax from your thumbnail to the middle front of the card.
Spin the card and allow it to move freely on the end of the suspended string.
Catch the string up in the fingers of one hand, and manipulate its position to make the card move about while "floating."
Catch the card with your free hand, secretly transfer the magician's wax back to your thumbnail, and hand the card back to an audience member to inspect.