How can I effectively buff polyurethane by hand to achieve a smooth and glossy finish?

To effectively buff polyurethane by hand for a smooth and glossy finish, start by sanding the surface with fine-grit sandpaper to remove any imperfections. Then, apply a thin layer of polyurethane using a clean cloth or brush, following the wood grain. Allow it to dry completely before lightly sanding with a higher grit sandpaper. Finally, use a soft cloth or polishing pad to buff the polyurethane in circular motions until you achieve the desired shine.