How can I achieve a smooth finish when applying oil-based polyurethane with a rag?

To achieve a smooth finish when applying oil-based polyurethane with a rag, make sure to sand the surface beforehand to remove any imperfections. Apply the polyurethane in thin, even coats, using long, smooth strokes in the direction of the wood grain. Allow each coat to dry completely before applying the next one. Sand lightly between coats with fine-grit sandpaper to remove any bumps or imperfections.