Trace a gear pattern on a sheet of plasticard with a pencil. Draw guidelines using a compass and ruler to make sure the pattern is laid down correctly. Use the Sharpie to trace over the pencil once the pattern has been drawn correctly.

Cut around the gear with a pair of scissors. Start the scissors as close to the pattern as possible. Leave around an inch of extra plasticard around the gear pattern. Do not try to cut the individual gear teeth with the scissors. This could result in uneven or warped gears that will not fit together.

Cut the teeth out of the pattern with an X-Acto knife. Press down firmly in the corner of a gear tooth and rock the blade outward to start the cut. Continue the cut until the blade has cleared through all of the plastic. Repeat the process, always beginning at the corner of the gear tooth and moving outward. Match the pattern exactly.

Sand the teeth of the gears gently. Remove any excess plastic from the gear teeth. If any teeth have been cut too large, refine the shape with the sandpaper. Wrap the sandpaper around a thin bar of metal or a metal ruler to give the paper shape and allow for easier sanding. You may have to cut the sand paper to a smaller size so that it is easier to work with when folded.

Clean the gears and match them together to make sure they fit. Place a pencil or other long cylinder in the gear and spin it against the other gear to ensure they are in working order.