Set up the floor buffer with a medium refinishing screen. Turn on the machine. Push it over the floor in the direction of the boards, then pull it back. Continue, forward and back, taking off the top layer of the gloss finish. Do the whole floor, keeping the buffer moving at all times.
Vacuum up the dust.
Remove the medium refinishing screen and replace it with the fine refinishing screen. Screen the floor again in the same manner, getting the floor smooth and dull. Vacuum the dust.
Spread polyurethane onto the floor surface with a paintbrush, in a thin, even coat, working with the direction of the floorboards. Start at the far corner of the room and work your way toward the entrance.
Let the polyurethane dry for six hours. Dull the shine with fine sandpaper, buffing it by hand. Vacuum the dust.
Brush on a second coat of polyurethane, in the same way you did the first. Let it dry.
Sand the second coat of polyurethane. Vacuum the dust. Apply a third coat. Let it dry 48 hours.