According to Make, a Van de Graaf generator can be built in 10 steps. They are: build the base, prepare the motor, make the collector, prepare the top roller, make the top assembly, prepare the body tube, add the brushes, cut an 8 inch stranded wire, adjust and assemble and connect the motor.
The supplies needed for this project are as follows: A wooden dowel about 1 inch in diameter, four AA batteries, a glass fuse about 1.25 inches long and 0.25 inches in diameter, a large rubber band at least 0.25 inches wide and 12 to 18 inches long, a 12-oz. aluminum soda can, thick stranded wire that is 14 gauge or 2.08 square millimeters, a 4xAA battery pack with 9V connector straps, 9V battery snap connector, 1.25 inch PVC coupling, 1.5 by 1.25 inch PVC bushing, 1.25 by 1.25 by 1 inch PVC tee and 1.25 inch PVC pipe about 18 inches in length. The tools required are as follows: a can opener, a drill and drill bits, a handsaw, a hot glue gun and glue, a knife, pliers, a soldering iron and solder, wire strippers and a candle.
To build the base, stand a 3 inch by 1.5 inch PVC coupling on its wide end, and insert a 1.5-inch by 1.25-inch bushing. Cut a1.25-inch PVC pipe about 2 inches long and insert it into the bushing. Don't jam them together; press hard enough that they are joined, but loose enough to disassemble. Connect the tee to the 2-inch length of pipe. Wrap the motor in electrical tape so it fits snugly into the 1-inch side outlet of the tee. Create the bottom roller by wrapping the motor shaft in tape until it reaches about 0.5 inches in diameter. Split a length of electrical tape in half lengthwise, and wrap the split tape around the middle of the roller.
Remove the top of a clean, dry 12-oz. aluminum can with a can opener. Take the ends off a glass fuse using a candle and two pliers. Grip one end of the fuse with the pliers, and hold the other end over the flame. Use the second pair of pliers to remove the hot end cap. Repeat with the other end of the fuse. Remove any wire inside the fuse with the fliers. Clean soot from the glass tube with a cotton swab and alcohol. Keep the fuse dry and free of oils from your hand. Cut a 5-inch length of PVC pipe and push it into the 1.25-inch coupling. About 0.5 inches away from the coupling, drill a hole for the dowel through the pipe and through the other side. Ensure the holes are as level as possible. Drill a hole for the top brush. This hole needs to be large enough for the stranded wire, and located halfway between the two dowel holes, the same distance from the coupling. Cut the dowel to 1.75 inches long and put the dowel through the first hole, the glass tube, the rubber band, and the second hole. Use hot glue to secure the dowel to the PVC so it doesn't slip out.
To determine the length of the body tube, put the motor in the tee, loops the end of the rubber band over it and hold the top assembly up. The belt should have a little tension, but too much tension wastes power. Measure the distance between the top coupling and the top of the tee, and add 2 inches so the pipe is long enough to fit into the couplings. Cut the pipe to this length. There is already one hole drilled for the brush at the top, so drill a similar hole on the opposite side of the tee, large enough to fit the stranded wire. Cut a 12-inch length of stranded wire. Strip 1 inch of insulation off one end and 0.25 inches of insulation off the other end. Insert the end with 0.25 inches stripped through the hole in the tee until it is about 0.5 inches from the bottom roller's surface. Tape the wire down on the outside of the pipe with electrical tape.
Cut an 8-inch length of stranded wire and strip 1.5 inches of insulation off one end and 0.5 inches off of the other. Insert the end with the 0.5 inches stripped into the remaining hole in the top assembly so the uninsulated wire is about 0.25 inches from the rubber band, then tape the insulated section against the outside of the pipe to secure it. Untwist the stranded wire inside the tube and spread it out a little, keeping it close to the rubber band but not touching. Bend the other end of the wire over the top of the assembly and secure it with electrical tape. Insert the motor into the tee. Drop the rubber belt from the top assembly through the body. Insert the body tube into the top assembly. Pull the rubber band down and loop it around the bottom roller. Insert the body tube into the top of the tee, so the brushes are on opposite sides. Ensure the top and bottom rollers are aligned with each other. Hold the assembly up and look through the body, checking to see that the top and bottom brushes are close to the belt but not touching it. Also ensure the belt isn't twisted or so loose that it hits itself.
Put the batteries in the battery pack and snap the 9V connector onto the battery pack. Press each wire against the motor terminals. The motor should spin. The brushes should point at the moving belt on the side where it exits the rollers. If this is backward, swap the motor power wires. Unplug the battery pack, and solder the 9V connector's leads to the motor terminals. You can use the battery snaps to connect and disconnect the motor. Place the aluminum can on top so the bare wire presses against it. Attach the battery pack. A charge should build in the can.