To preserve a newly stained piece of wood for as long as possible, the stained piece is often covered with a polyurethane finish. This finish creates a hard surface over the wood that absorbs the nicks and scratches that come with normal use. If you need to remove the polyurethane while not affecting the stain beneath, you'll need chemical help in the form of a homemade stripper formula.
Pour equal parts of lacquer thinner and denatured alcohol into a small empty paint can. Mix with a wooden stir stick, scraping into the mixture any lacquer from the base and sides of the can. The mix will serve as your polyurethane stripper--strong enough to remove the finish but unable to penetrate the wood and disturb the stain if quickly removed.
Apply the mixture onto the polyurethane finish, covering the stained wood. Use a paintbrush to apply it evenly over the wood, slightly overlapping brushstrokes for complete coverage. Wait about 10 seconds for the mix to dissolve the finish.
Remove the mix immediately, using #0000 steel wool to wipe away both stripper and polyurethane.
Wash the wood with a wood cleaner to remove any stripper residue. Rinse with warm water and pat-dry the surface with a clean rag.