Installing the Plate
Locate and turn off the breaker to the circuit your light will connect to. Pull the wires out from the wall as far as possible. Remove the wire nuts if necessary and thread them through the hole in the center of the plate.
Position the plate against the wall and mark two corners with a pencil for alignment. Mark the holes in the back of the plate onto the face of the siding. Drill through the siding to create 1/8-inch pilot holes for each marked screw position.
Replace the plate and drive one 3-inch treated deck screw through each hole in the back of the plate into the face of the wall. Drive the screw in until the you have pulled the plate tight to the siding.
Installing the Light
Fit each wire into the jaws of wire stripping pliers, with about 1 inch of wire on the outside of the pliers. Close the pliers firmly to cut through the insulation and pull toward the end of the wire to remove it. Do this for all wires coming from the wall.
Remove the wire nuts from the leads on your light fixture. Match the wires by color: white to white, black to black and green to green. Fit a wire nut on each pair of wires and twist the nuts clockwise to tighten.
Position the base of the light fixture on the plate and align the mounting screw holes with the holes in the plate. Drive the two mounting screws through the base plate of the fixture into the wall plate, using a screwdriver.
Caulk along the outside edge of the plate with silicone caulk. Apply it with a caulking gun and smooth it with a dampened finger to complete the installation.