Wet the rock and use the 80-grit sandpaper to roughly shape and round the corners of the stone. Keep a bucket of water nearby so that you can continue to wet the rock as needed. Continue this until the rock has been shaped to your satisfaction.
Use the 180-grit sandpaper to further refine the shape of your rock and to remove any heavy scratches. Make sure to keep the rock wet throughout this process as well.
With the rock still wet, use the 600-grit sandpaper to get rid of any lighter scratches on the rock's surface. Rinse the rock with water often during this step.
Place a bit of powdered polish on a damp denim cloth. Polish the rock with the cloth until you are satisfied with the result. Depending on the rock, you may have to keep adding polish to the damp cloth at regular intervals.