Go to the fuse box and trip the circuit breaker that supplies electrical power to the room where you will be installing the Firex FXW-1.
Place a ladder against a wall in the room where a light fixture is located. Remove the screws surrounding the light fixture, using a Phillips screwdriver. Put the screws in your pocket. Pull the fixture off the wall.
Unscrew the screw-nut connectors on the twisted pairs of wires that have now emerged from the hole behind the fixture. Put the connectors in your pocket.
Push the wires attached to the detector through the mounting plate. Twist the already-stripped end of the black wire attached to the Firex to the black wires coming out of the hole. Twist the already-stripped end of the white wire attached to the Firex to the white wires coming out of the hole.
Aim a flashlight into the hole to find the single screw on the bottom left or right corner of the electrical junction box. Loosen the screw, using the Phillips screwdriver.
Twist the already-stripped end of the yellow wire attached to the Firex around the screw and tighten the screw. If the Firex is not going to be interconnected to other alarms that have been installed, cover the end of the attached yellow wire with strips of electrical tape. If the Firex is to be interconnected with other alarms, twist the end of the yellow wire from the Firex around the end of the yellow wire coming out of the hole. Screw a screw-nut connector over each of the twisted pairs of wires.
Place the Firex mounting plate against the wall, next to the light fixture. Screw wood screws through the holes in the sides of the plate and into the wall, using a Phillips screwdriver.
Place the bottom of the Firex against the grooves on the plate. Twist the Firex onto the plate, using a clockwise motion.
Reattach the light fixture to the wall. Put the ladder away. Restore the electrical power at the fuse box.