Use a screwdriver to unscrew the trucks from the deck. Most truck screws feature a Phillips head design, but the occasional set of trucks come with an Allen head design. Use a pair of pliers, wrench or ratchet socket to hold the bolt underneath the board in place. Twist the screwdriver to remove the screw. Repeat on each screw--8 total--and remove the trucks.
Remove the wheels from the trucks. Use a 1/2-inch socket wrench to loosen and remove the bolt on each wheel. Take the wheels and spacers off the truck. Put the spacers in a safe spot so you don't lose them.
Remove the bearings from the wheels. This can be done in several ways, but the easiest is to use your truck to pry the bearings out. Place the bearing around the tip of the truck and slowly pry the bearing out by pushing down and back with your hand. Once the first bearing of each wheel is off, you can simply push the other bearing out from behind with your hand, screwdriver or the butt of the truck.
Remove the grip tape. This is optional, of course, but necessary if you're looking to completely disassemble a skateboard. Use the heat from a hair dryer to soften the glue of the grip tape. Then take a razor blade and lift an edge of the tape. Pull up to remove. If the grip tape breaks, use the razor again and apply more heat as needed.