Vacuum the plywood thoroughly, picking up all dust and debris from the cracks and holes. Mop the surface with household cleanser. Let it dry completely.
Pour 1 to 2 inches of water in your bucket. Add enough powdered floor leveler to just cover the water.
Mix the solution with your drill and mixing bar. Add more water and powdered leveler, alternately, until you have enough to fill the low spots and gaps in the floor, and it's the consistency of thin soup.
Pour the leveler slowly over each hole, gap or low spot on the floor. Start at the corner of the room that's furthest from the doorway, so you can work your way out. Pour just enough over each area to fill it and let it spread out level.
Allow the leveler to set for 24 hours. Thoroughly sweep the floor to remove any surface dust. The floor is now ready for the top flooring.