Insert a stopper into the hole in the jig. Do not attempt this without the jig; holding the stopper with your hand will result in injuries!
Chuck the 5/16" drill bit into the drill press. Center the bit on the stopper, turn on the drill press, and drill through the stopper. You want a hole completely through the stopper. Most likely, you'll have to unscrew the stopper off of the drill bit.
Remove the stopper from the jig, and using the box knife or razor blade slice from the top to the bottom of the stopper with the point of the knife just inside the hole in the stopper. Later, you'll fit the stopper over the cord of the light string to act as a grommet.
A great tip from users 79spitfire and prof_jellis: Putting the rubber stopper in the freezer overnight will make it easier to drill; drill slowly so the bit doesn't get clogged with melted rubber. www.instructables.com/id/a-little-brown-egg-in-Maine%3a-terra-cotta-smoker/step4/Top-it-off/