Remove the drywall panel above your doorway or window to gain access to the header. Use your crowbar to pry the edges of the drywall panel away from the studs and header, and use your screw gun or hammer claw to remove the drywall screws or nails. (Reference 5)
Use your hand-held router to notch a groove into the header wide and deep enough so your power wires can pass through. Only cut as much as necessary, and start with a shallow and narrow cut, expanding as needed. Attempt to fit the wire after each attempt to avoid over cutting. Once the wire fits into the groove, you must cover and protect it.
Install flat metal nail-protector plates over the wire, and the notch to prevent nails and screws from being driven into the wire. Each plate has metal spikes at either end. Use your hammer to drive these spikes into the header, and use your screw gun to install drywall screws through the metal plate screw holes.
Install a new drywall panel over the header and studs. Use your screw gun and drywall screws to fasten the drywall panel in place.