Cut two sheets of aluminum foil to a length that overlaps a jar's circumference by 1/2 inch. Trim both sheets of foil to 3/4 the height of the jar.
Wrap one sheet of foil around the outside of the jar and tape it securely to the jar. Insert a second sheet of foil inside the jar directly opposite of the outside sheet and tape it securely to the inside of the jar. The two sheets of foil act as the plates that will hold charge. The wall of the glass jar between them is the insulating dielectric.
Puncture a small hole through the plastic lid of the jar and insert approximately 4 inches of a 6-inch copper wire through the top of the hole in the lid. The plastic lid acts as an insulator to hold the copper wire up and away from the outside of the jar to prevent a short circuit.
Tape the long end of the copper wire securely to the foil inside the jar. Screw the lid shut.
Tape a 6-inch piece of copper wire to the foil on the outside of the jar. The Leyden jar is now complete and ready for charging using the two copper wires as leads.