Mark the locations of the studs in your closet to which you plan to attach shelves. Draw continuous, horizontal lines down the lengths of your planned shelves, checking the line with a level; if and when your lines reach the sides of the closet, draw lines across the corner and extending to those sides.
Use the cutting guide to cut 2-inch cleats from some of your boards; the cleats should be as long as your planned shelves. Measure the side lines you drew, subtracting the thickness of the cleats you just cut. Cut cleats to match these lengths.
Install your cleats directly underneath the lines you drew to mark the studs, as well as the side lines, and then use your 3-inch wood screws to attach the cleats directly to the studs.
Cut your remaining boards to the planned length and depth for your shelves.
Dry-fit your boards to your cleats, and check again with the level that everything is even.
Drill pilot holes at regular intervals across the back of the top of your shelves. Using these pilot holes, screw the shelves into the tops of your cleats to complete the project.