Unscrew the shower door from its hinges. Take it outside or to a well-ventilated area. Spread a drop cloth on your work surface.
Cover metal or any non-glass parts of the shower door with painter's tape. Put on a pair of goggles.
Shake the can of frost spray for one minute. Hold the can 8 inches from the exterior of the glass door, at the top left hand corner of the door. Move the can from left to right, creating a light mist of frost on the door.
Apply a second coat of frost after the first coat has had 24 hours to dry. Allow the frost to cure for a full two days before re-hanging.