Remove any knobs, handles or brackets from the furniture with a screwdriver. If the furniture piece is a chair, remove the seat cushion by taking off the seat pan from the frame. The screws for the seat pan are located under the seat.
Sand the surface of the wood with a flexible 80-grit to 120-grit sanding block. Remove the all of the finish. It should come off quickly. Shellac is thin and there will be no paint to contend with. If the sanding block leaves visible scratches in the surface of the wood, use higher-grit sanding blocks to remove the scratches. If you choose, you can start with a higher-grit sanding block. The higher the number, the finer the sanding surface. Therefore, if you use a higher-git sandpaper, it will take longer to sand but it will leave less micro-scratches.
Wipe the wood down with mineral spirits or microfiber cloth to remove all of the dust from sanding.
Place shellac flakes into a bowel and mix in a small amount of denatured alcohol. Apply the shellac to a microfiber rag and rub a coat of shellac onto the furniture. After approximately 30-minutes, apply a second coat of shellac. Allow 48-hours drying time. If desired, add a small amount of stain to the shellac to change the color of the wood.
Reattach all of the hardware to the furniture once completely dry.