Locate the edge of the wooden ceiling truss under the drywall with a stud finder. You will attach the electrical box to the truss where you plan to install the light fixture since the light fixture attaches to the electrical box.
Place the electrical box against the ceiling so the side of the box with the mounting holes in it lines up with the edge of the truss.
Trace the electrical box onto the ceiling with a pencil.
Shove the keyhole saw into the drywall and cut along the line you scribed with the pencil. Remove the unwanted section of drywall from the ceiling.
Place the electrical box into the ceiling so it rests flush with the drywall and butts up against the side of the truss. Secure the box to the truss by driving the supplied nails through the side of the box with a hammer or by driving in the supplied screws with a screwdriver.