Place the front of the bracket against the drywall at the installation location. Set a torpedo level on the top edge and adjust the box to center the bubble. Poke a pencil through the reference hole near each of the corners and make a mark on the wall. Remove the bracket.
Draw lines between the marks to form a rectangle, using a ruler and a pencil. Double-check the level of the top and bottom lines. If they aren't level, the cover plate for the box will be askew.
Cut around the rectangle, using a drywall saw. Remove the cutout.
Fit the bracket into the hole and turn the screws in front clockwise, using a screwdriver. The mounting brackets will automatically align against the back of the drywall as you do this, and tightening the the screw will secure the bracket.