Mark cut lines on the cord at the desired location for the switch to ensure the switch assembly will cover exposed wires.
Score two cuts around the perimeter of the exterior vinyl or rubber sheathing of the extension cord with a utility knife, perpendicular to the length of the cord, slightly shorter than the length of the switch assembly. Only cut through the exterior plastic sheathing to the paper or cloth insulation layer, so as not to score internal wires.
Remove sheathing and insulation wrap from opening to expose internal wires. There will be three individual wires within the extension cord sheathing. Typically, one is white and one black, with a green ground wire.
Cut the black or white wire at the midpoint of the exposed section of cord and strip sheathing from both wire ends to expose approximately ½ inch of bare wire.
Attach the two ends of the cut wire to the two copper screws on the switch and tighten to secure.
Wrap electrical tape around the cord, beginning at approximately 1/2 inch from the cut opening in the cord, to cover exposed wires and both connections on the switch. Replace the switch cover and tighten the screws.
Plug the cord into the wall outlet. The switch will now control electrical flow to an appliance or tool.