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Using a good quality paint brush, apply the polyurethane to the stair treads. Work with the grain of the wood, using nice, even strokes. The goal is to achieve a smooth and consistent coat without drips or brush marks. For the final pass on each tread, make one long, continuous stroke with the grain to even out the finish. The presenter recommends applying three coats of polyurethane for maximum durability. Allow each coat to dry for a couple of hours (refer to product instructions for exact drying times), then lightly sand the surface with a fine-grit sandpaper (e.g., 220-grit) and wipe clean with a tack cloth before applying the next coat. This inter-coat sanding helps to smooth out any imperfections and ensures better adhesion for subsequent layers.