Sand all faces of your lumber and wipe clean. Apply a coat of stain in a well-ventilated area. Allow to dry, then apply a second coat.
Locate the wall studs on the wall where you want to mount your desk unit. Standard spacing on wall studs is 16 inches on center.
Mount one 2-foot bracket on one wall stud. Place it so its upper arm is 30 inches off the floor. Fix it in place using 3-inch wood screws and power drill.
Mount the other 2-foot bracket to a wall stud two studs away from the one you already mounted. Use your level to confirm that the upper arms are even with each other. Screw it in place as you did the other.
Set the 24-by-40 wooden sheet on top of the brackets, with 3 inches of wood overlapping on either side. Screw it to the brackets using 1/2 inch wood screws. If you screw in from the bottom, the screws won't mar your work surface.
Mount one 12-inch bracket to a wall stud at least 18 inches above the surface of your desk. Use 3-inch wood screws. Mount a second 12-inch bracket to an adjacent wall stud, using your level to keep the upper arms level.
Place a 20-inch wooden sheet on top of the brackets, with 2 inches overlapping on either side. Screw in place using 1/2-inch wood screws driven from underneath.
Repeat Steps 6 and 7 to mount the other shelf in an appropriate area. You can add more than two shelves if you have the supplies and inclination.