The key to a good finish is starting with a nice smooth surface, since your finish can only be as smooth as your wood surface. Make sure all dust, debris, oil, and grease is removed before applying any polyurethane. When applying polyurethane make sure you are in a dust free area with proper ventilation.
The first step to applying a coat of polyurethane is sealing the surface of the wood. Mix 2 parts polyurethane with 1 part mineral spirits. Using a natural bristle brush apply long, even strokes and make sure to catch any runs. Allow the sealing coat to dry for 4 to 6 hours.
Within 24 hrs of applying the sealing coat, apply your first coat of undisputed polyurethane. Use long, even strokes to coat the surface. Be careful not to apply too much as the finish will start to run. Wait 5 minutes after you applied the coat and inspect for any drips. Let the coat dry for 24 hours before applying your final coat.
Apply your final coat in the same way as you applied the first coat and let it dry for 24 hours.
Now its time to inspect your surface for any blemishes. If you find any dried drips, cut it away with a razor blade. Any small blemishes will disappear with 400 grit wet sanding paper. Make sure you use enough water to lubricated the sandpaper. Do not sand too deep because it can make a finishing nightmare, just enough to remove the blemishes.
For the final step you have two choices. If all you had to do was sand the final coat skip to the polishing step. If you found you had drips and had to cut them away, then apply a third coat of polyurethane in the same manner you did the other two coats.
When you are ready to polish the sanded surface of polyurethane, make sure it has been at least 48 hours after the final coat was applied. Dampen a clean cotton cloth with water and apply automotive rubbing compound in circular motions to the surface. After rubbing, let the surface dry and buff the finish with clean cloth. If you find the finish is still a bit cloudy apply a second coat of automotive rubbing compound in the same manner as the first.