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Make a fist with your hand.  Release your index finger so that it points straight out.  Release your thumb so it points straight upward.  This is the basic structure of the one-handed gun. Aim the gun by pointing your index finger toward where you want the rubber band to go.  Don't aim it at a person or anything that could be harmed by a flying rubber band.
Turn your hand into the shape of a gun. Aim your gun.