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If you're staining a dark piece of wood a lighter color or you need to take off a lacquered top coat, sanding may be a good option. Sanding is the quickest way to take existing stain off of a piece of wood, especially if you're working with a small piece of wood or a large, flat surface with no detail. Sanding is also a good choice if you don't want to work with chemicals. Start with a rough piece of sandpaper, like a P80 grade, for the first pass over the wood, then go over the area with a medium-grit, such as a P150. If you need to, you can finish with a fine grit, like P220. Graduating to a finer sandpaper will help keep you from scuffing the surface of the wood too much. Whether you're using an electric sander, a sanding block, or just a piece of sandpaper, hold the sandpaper so it's flat against the surface of your wood as you're working. This will create an even finish. Otherwise, you could sand unevenly, wearing into the wood and creating a light spot that will show through the stain. Although you won't be working with dangerous fumes while you're sanding, you will be putting a lot of small particles into the air, and those can irritate your lungs if you breathe them in. A dust mask will help protect your airways while you're working. You can usually find dust masks wherever home improvement supplies are sold. After you're finished sanding, you'll want to make sure you don't leave any dust or residue on the surface of the wood. Otherwise, it will get caught in the stain and cause a gritty finish.

Summary:
Sand the wood if you are only working with a small piece. Work from a rough grit to a fine grit. Keep the sandpaper or sander flat as you work. Wear a dust mask while you're sanding. Wipe down the surface of the wood with a wet cloth to remove any dust.