Remove the wheel trucks with the appropriately sized wrenches and screwdrivers for your skateboard; set the hardware aside. Depending on your skateboard, the trucks might be fastened with a combination of screws, nuts and bolts.
Peel away the grip tape on the top of the board. You may need a small amount of adhesive remover to loosen the glue.
Sand the edges with an 80-grit sanding block to achieve a rounded shape. Alternatively, you can use a piece of sandpaper wrapped around a wooden block or a palm sander outfitted with a piece of 80-grit sandpaper.
Brush the dust away with a clean brush. A clean, wide paintbrush works well for this.
Repeat the process of sanding the edges and brushing the dust away with the 120-grit, 180-grit and 220-grit sanding blocks. When you finish, the edges should be smooth and perfectly rounded.
Sand the top and bottom of the skateboard with the 80-grit sanding block with long even strokes, following the grain of the wood. Sanding against the grain will prevent you from achieving a smooth surface.
Run your hand over the board to ensure that the surface is sanded evenly. Continue sanding any areas of inconsistency until the entire board is even.
Repeat the sanding process on the top and bottom of the board using the 120-grit sanding block, then the 180-grit sanding block and finishing with the 220-grit sanding block.
Paint a graphic design on the underside of the board and coat with two to three layers of protective polyurethane.
Measure and apply a new sheet of grip tape to the top of your board, being sure to push out all air bubbles as you go. You can purchase pre-measured sheets of grip tape with graphics from many skateboard shops and online retailers.
Replace the trucks on the bottom of skateboard.