Run a stud-finder along the side walls of your closet or alcove. Mark the locations of the studs with a pencil.
Measure the vertical height of the shelving area. Divide this measurement by the desired number of shelves to determine the spacing of your shelves. Double-check that the shelves will not be too close together---most household shelves should be at least 24 inches apart to accommodate a variety of storage needs.
Measure the depth of the side walls. This measurement will determine the length of the side rails you will cut to support your scrap wood shelves.
Put on your safety glasses and cut two lengths of 2-by-2-inch dimensional lumber for each desired shelf. These pieces will be serve as the side rail supports for your shelves.
Mark the walls at the points where the shelves will sit. Use a spirit level to level each side rail at these points. Check the marked stud locations, and screw the side rails into the studs with 3-inch wood screws.
Cut the platforms for each shelf from ½-inch plywood with a circular saw. Fit the shelves onto the side rails and attach each shelf to its side rails with a pair of 1-inch wood screws.