Mark the wall at the height of the bottom of the cabinet shelf. Place a carpenter's level on the mark and scribe a horizontal line on the wall. Use a stud finder to locate the wall's framing studs. Mark their positions a few inches below the horizontal line.
Position a 1-by-3-inch plank on the wall so its upper edge is aligned to the horizontal line. Drill pilot holes through the plank, into the wall studs. Temporarily attach the plank to the wall, using drywall screws. The installed support rail will hold the cabinet level and in place as you install the cabinet.
Place the bottom of the cabinet on the support rail. Ask an assistant to hold the cabinet in place. If necessary, locate the framing studs above the cabinet, and mark their positions on the top of the cabinet. Measure the distance from the left and right edges of the cabinet to the centers of the studs. Transfer these dimensions to mark the stud positions on the nailing strip inside of the cabinet.
Drill pilot holes through the nailing strip, into the framing studs in the wall. Insert a screw into each pilot hole, and screw the cabinet to the wall. For extra support, drill pilot holes in a vertical line about 4 inches apart beneath the installed screws. Screw the cabinet to the framing studs at each pilot hole. The screws will be concealed by the installed microwave oven.
Remove the support rail.