Secure the cord you are going to upgrade; make sure it is not plugged into the wall socket. Cut the cord in half or where you want tho install the switch. Take scissors and cut between the two sides of the extension cord; peel them apart so you have two separate wires. Repeat this process for the other half of the cord.
Strip back 1/2 inch of insulation from each separated wire. Open the new switch by unscrewing it with either the flat-head or Phillips screwdriver. The inside of the switch should have a brass side and a silver side.
Place one side of the wire onto any side of the switch, as long as each cord half wires are on the same side. Screw each wire to a different-color screw. Repeat the same process with the other half of the cord. Make sure that each wire is on one screw, and that each cord half is on a separate side.
Replace the cover for the switch, making sure that the wire is enclosed over the insulation. Plug the new cord into an appliance and into the wall socket. Turn on the switch.