Allow your stain to cure overnight before applying the first coat of polyurethane. Use a medium-width soft bristle brush. Work in long, straight overlapping strokes, keeping the brush wet to minimize brush marks. Cover the entire surface, working to prevent runs and drips. Allow the finish to harden overnight. Pour 1/2 inch of water into a plastic pan and add two drops of detergent to break the surface tension, allowing the water to soak in better. Place a rubber sanding block with 320-grit wet/dry sandpaper on it into the pan. Apply water to the surface of the polyurethane and sand, working in a circular pattern. Wipe the surface with a clean cloth every so often to gauge progress. Work until the surface has an equal dull haze to it.