Use the sanding sponge available at the hardware store .........a foam sponge with abrasive bonded to the surface.........
Cover the floor with a dropcloth..........
Use a bucket of water to rinse the sponge.......wring out until damp .....then sand the finish..........rinse the sponge often and wring nearly dry.........the water "melts" the compound smoothing the surface and filling small defects........continue until you match the surrounding areas........