Pull the fuse or trip the circuit breaker in the fuse box that supplies power to the lighting fixture that will be converted into a motion sensing light.
Remove the screws from around the lighting fixture with a Phillips screwdriver -- if the lighting fixture is on the edge of a roof or high up on a wall, use a ladder to reach it.
Pull the fixture off the wall. Twist the screw tabs counterclockwise to remove them from the pair of wires they are around. Separate the wires from the back of the lighting fixture and from the electrical junction box inside the wall cavity from each other.
Screw the motion sensor to the wall, next to where the wall cavity is, using wood screws and a Phillips screwdriver.
Wind the exposed ends of the wires attached to the motion sensor to the wires attached to the lighting fixture and the electrical junction box in the following manner: the white wires from the motion sensor, lighting fixture and electrical junction box together; the black wire from the motion sensor to the black wire from the electrical junction box; the red wire from the motion sensor to the black wire from the lighting fixture. Twist a screw tab over each of the twisted-together wires.
Place the lighting fixture against the wall. Reattach the screws. Restore power to the lighting fixture.