Thread the connector clockwise onto the twisted wires, being sure no bare copper is exposed at the bottom. Pull gently on the connector to be sure it’s secure. You can wrap electrical tape around the wires and cap for extra security or just push the splices into the back of the box and attach the cover plate.
With the 2x4 and box assembled, hold them between the joists so the outlet box presses against the back of the ceiling. Trace the outline of the box with a pencil to mark the hole. Remove the assembly and cut a hole along the trace line using a drywall saw for drywall or gypsum board plaster and a reciprocating saw for wood lath plaster.
If permitted, new electrical wiring may be run behind baseboards to serve a new wall receptacle, or to a switch and new ceiling light fixture. To pass the cable through the wall studs a notch is cut in the wood and a metal shield, sometimes called a nail guard, is fastened over the notch to protect the new wires from nail and screw punctures.
Fit a nail guard into each notch, covering the new wires. Drive the plates down tightly making sure they are flush and not protruding from the stud surface.