Kevin, If you put down new plywood then screw down the panels, dont nail in down. When you screw down your panel sink the head of the screw below the surface of the panel, you can then fill with a filler and sand smooth. If you nail ( much more simple)you will have problems later with squicky floors, All timber moves, so the nails that are there will come loose The sides of the panels, if you plane a "V" along the edges you can fill that with filler again sanding smooth. Hope it works. John 2000.