Pour the mineral oil into a large, dry glass jar. Select a jar with a wide mouth. Leave approximately 1 inch of air space at the top of the jar. Measure the height (H) of the oil in the jar using the ruler.
Cut the paper into two equally sized rectangles. The width of each rectangle should be (H - 1) inches. The length of the rectangles will be limited by the length of the paper. Measure the length (L) of the rectangle.
Cut two aluminum foil rectangles. The length of the rectangles should be (L – 1) inches, while the width is (H – 2).
Center and tape each foil rectangles onto a paper rectangle. If centered correctly, a frame of 0.5 inch of paper will be visible around the foil rectangle. Position the foil-paper constructions in landscape orientation, with the foil on the upper side.
Cut two pieces of wire, each 6 inches long. Strip 1 inch of insulation from either end of each piece of wire.
Make a tiny hole in the foil at the top left corner of one foil-paper construction. Also make a tiny hole in the foil at the top right-hand corner of the other foil-paper construction. Tuck the bare end of a piece of wire into each hole. No wire from which the insulation has been stripped should protrude from the hole in the foil. Make sure that the wires do not tear through the paper or the foil. Secure the wires in place with tape applied to the paper close to the hole in the foil.
Place the second foil-paper construction on top of the first foil-paper construction. The foil should be facing upward. Wires should be visible at the top right and left corners of the stack of foil-paper constructions. Tape the top construction to the bottom construction so the stack does not slip when moved. Tightly roll the stack from the left edge to the right edge. Loosely rolled capacitors perform poorly. Tape the roll closed.
Lower the rolled structure into the jar of oil. The wires should drape over the lip of the jar. The roll structure must be fully submerged to avoid break down of the capacitor during use.