Step 1: Construction
In this jar there are five main parts. the inside foil, the outside foil, the inside wire, the outside wire, and the jar itself which must be an insulator. basically if you build up a positive potential in the inside foil, and keep the outside foil grounded, it will want to make the charge on the outside equal that of the inside, so a spark will jump from the grounded outside to the inside.
To make it all you need to do is glue some aluminum foil to the outside of the jar (which needs to be glass or plastic), and tape a wire to it, glue some more foil into the inside and tape some wire to that. drill a hole in the lid of the jar and put the wire connected to the inside through it. screw on the lid and you are done.