When sanding any surface, there is one cardinal rule that you must always follow: work in the direction of the grain. Sanding against the grain will result in a rougher, more uneven finish. It can also damage the surface of your material, so it's important to be mindful of which way the grain is running before you start sanding. Once you've determined the direction of the grain, simply sand back and forth in long, even strokes until you've achieved the desired results.