Lightly buff the existing floor with 200-grit sandpaper by hand, just enough to dull any shine. Thoroughly clean and vacuum the floor. Go over the surface with your tack cloths to pick up any remaining dust.
Put on your rubber gloves. Soak a few rags in rubbing alcohol. Wipe down the entire surface of the floor. Let it dry.
In a bucket, mix a 50-50 solution of shellac and rubbing alcohol. Mix just enough to cover the floor in one thin coat.
Use your brush to apply the "cut'' shellac (diluted with alcohol) to the floor, starting in the furthest corner from the doorway. Brush it on in the direction of the floorboards. Brush it slowly and carefully, to avoid creating bubbles.
Let the first coat dry for about six hours. Lightly sand it by hand with your 200-grit sandpaper, buffing it just enough to dull the shine of the shellac. Vacuum up the dust. Follow up by going over the whole floor with tack cloths.
Apply a second coat of "cut" shellac, applying it as in Step 4. Let it dry and buff it with your sandpaper. Clean up the dust.
Apply a topcoat of "uncut" shellac (don't mix any alcohol into it), brushing it on in the same fashion as the earlier coats. Let the topcoat dry for 24 hours before using the floor.