Lay a 30-by-50-inch desktop wrong-side up on your work surface. Position four plate-mounted desk legs approximately 6 to 7 inches in from each corner of the desk top. The plate mount should be flush against the underside of the desk.
Secure the legs to the desk with the accompanying screws of the desk legs through the screw holes in the plate mounts into the underside of the desktop.
Stand three pieces of 50-inch 1-by-12 lumber up on a 1-inch edge spaced 12 inches apart. These will be the shelves of the desk.
Abut two planks of 38-inch 1-by-12 lumber over the ends of the shelves. One end of the 38-inch planks should form a 90-degree corner with the top shelf plank, and the other end will have a 14-inch overlap from the last shelf.
Glue the shelf ends in position to the side of the 38-inch planks, then secure them with three 2-inch screws driven through the 38-inch plank sides into the ends of the shelf planks.
Finish the shelves as well as the desk top and legs with the desired wood finish and let dry before assembling.
Stand the shelf unit up at the back of the desktop. The ends of the 38-inch planks should fit snugly over the sides of the desktop, and be flush with the underside of the desk top. This will make the bottom shelf approximately 12 inches up from the desktop.
Clamp the shelf unit in place onto the desk top, then secure it to the sides of the desktop with four evenly-spaced 2-inch screws through the 38-inch plank ends into the sides of the desktop.