Locate the breaker box and find the appropriate breaker. Be sure that the breaker is off and there is no power going to the damaged wire.
Cut the damaged wire 3 inches in both directions from the penetration of the screw. Discard the damaged piece of wire.
Slide the freshly cut ends of wire into the junction box. Slip one wire through each end so that they meet in the middle of the box.
Cut away the rubber sheath to expose 3 inches of the smaller wires inside of both large pieces of wiring. Strip each of the smaller wires with your stripping tool to expose 1/2 inch of each wire.
Connect the wires with wire nuts. Connect black to black, red to red and white to white. Loosen the ground screw and twist both bare ground wires onto the screw and tighten it to hold them into place. Wrap each wire nut and the wires it holds with electrical tape to secure the wire nuts.
Slide the cover onto the junction box. Tighten the retaining screws on the outside of the box with your screwdriver. Tighten the clamp screws on either side of the junction box to hold the wires in place. Restore the power to the wire by activating the breaker.