Measure the width of the desk and divide the result in half. Place a mark on the underside of the desk to show where the tray will be installed.
Cut a piece of plywood the desired length and width of the tray, two pieces of plywood the desired length and width of the side pieces and two pieces of plywood the desired length and width of the back and front piece.
Attach the sides to the back piece of plywood with the nails or screws evenly spaced. Attach the bottom piece of plywood to the back/sides piece.
Attach hinges to the bottom of the drawer with grabber screws. Attach the other half of the hinge to the plywood piece that will be used for the front piece.
Attach a magnet on each of the front pieces of the drawer and tray to hold the drop-front piece when not in use.