Apply a commercial chemical stripper to the wood with a paintbrush. Do this in an area that is properly ventilated. Wait the required amount of time according to package instructions.
Scrape off the polyurethane with a plastic putty knife. The finish will dissolve into a goopy substance. Scrape this off into a bucket as you work. Clean your putty knife periodically as you work. Do not gouge at the wood; scrape very lightly.
Sand any remaining polyurethane with a fine-grain sanding block. Although chemical strippers usually get most of the finish off, they sometimes leave some behind.
Set the heat gun to its lowest setting. Do not use a high setting on wood.
Move the heat gun over a small area of the furniture. Hold the gun at least 6 inches from the wood. Do not let it sit in one area too long or it will scorch the wood. Wave it back and forth.
Scrape off the finish with a plastic putty knife as it melts. You again will be left with a goopy substance. Scrape this off and discard it in a bucket.
Start the hand sander above the furniture piece. If you turn it on while the pads are in contact with the wood, it could gouge the surface. Lower the sander onto the wood slowly.
Sand off the top coat with a fine-grit sanding pad. Do not over-sand. Make several light passes. You should not see the true color of the wood -- if you do, you've sanded too far down, past the polyurethane coat.
Wipe the furniture down with a tack cloth. Look for any missed spots and sand as necessary. Use steel wool to sand in detailed areas that the hand sander cannot reach.