You remove a flat, invitation-size envelope from your pocket and dump out an "invisible deck". You direct a spectator to freely "destroy" these imaginary cards until only a single card remains. Let's say the Queen of Diamonds.
The spectator tosses the invisible Queen of Diamonds toward the envelope. You. then slowly and cleanly remove the "one & only " card in the envelope. It's the spectator's "one &only" Queen of Diamonds.