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.
Leave the device hanging free so it doesn’t make contact with anything and test it to be sure the wiring is correct. Flip the switch on and turn the power back on at the breaker box to test the light circuit. Use a receptacle analyzer to test a receptacle for proper wiring. If there’s a problem with the wiring turn the power off again and correct the problem.
To install a fixture using a coupler, thread the rod into the center hole in the mounting strap and set the rod depth so the fixture base will fit snuggly against the ceiling when it’s installed. Measure the depth of the cover plate to estimate placement, precision isn’t necessary here, small adjustments can be made to the position after the installation by turning the coupler nut. Be sure all the wires are stripped for splicing, hold the fixture up and feed them through the threaded rod into the outlet box.
A junction box has several holes with removable plugs in the sides and back. Choose holes conveniently located for the cables you are splicing and remove the plugs. Have a cable clamp for each one and loosen the screws to open the collar. Pull about 8 to 10 inches of cable through the clamp and tighten the collar around it so it’s firmly held but not pinching the plastic.