Position the two 77-inch boards on edge, so they are parallel and 35 inches apart. Position two 35-inch boards perpendicularly between them so they are also on edge and 75 inches apart. You should have a 77-by-37-inch frame. Pre-drill screw holes through the 77-inch boards and into the 35-inch boards using your 3/32-inch drill bit. Drill three holes for each corner of the frame, 1/2-inch from each 10-inch end of the 77-inch boards, and into the 1/2-inch center of the 35-inch boards. Space the holes 3 inches apart. Screw the boards together using 12 3-inch screws to secure your frame. Pre-drill 3/32-inch holes for every screwing application in this project.
Lay the plywood on top of the frame and screw it to the frame using the 24 2-inch screws. Screw into the 1/2-inch center of the frames edges and space the screws 9 inches apart. This will hold your mattress.
Position the two 39-inch boards vertically, parallel and 77 inches apart. Set the 79-inch board on top of the two boards so the corners are flush, and screw the 79-inch board to the 39-inch boards using the same process as you did in Step 1. Use 6 screws for this task. This will hold your mattress frame.
Position the mattress frame vertically, so it rests on one of its 77-inch boards. Lift one end up and place a dowel underneath the frame so it is 2 inches from the corner. Repeat this process on the other side. The dowels should be centered under the 10-inch board and perpendicular to it.
Position the 39-inch boards around the frame so the 79-inch board is 1-inch above the frame. All of the edges between both sets of boards should be flush. Drill a 1-inch hole through the 39-inch board and through the 35-inch board on the frame. The hole must be parallel with the 77-inch boards, 2 inches above the bottom of the 39-inch board, and 2-inches away from the plywood on the frame. Repeat this process on the opposite side so the two holes are aligned. Place a 2-inch dowel in each of these holes to secure both sets of wood together. The mattress frame should be able to fold down horizontally with ease once the dowels underneath it are removed.
Fold the bed up so it is in its vertical position. Drill two 1-inch holes through the 79-inch board, and into the 77-inch board on the mattress frame. The holes should be along the 5-inch center of both boards, and 6 inches from each end of the board. Place the 4-inch dowels in the holes to secure the frame in this upright position.
Screw the 76-inch board to the top edge of the plywood, so the screws enter the 77-inch board. This board should be flush with the top edge of the frame, with its end 1/2-inch away from each 37-inch side of the plywood. Use the remaining four 2-inch screws, and space them 18 inches apart. It will ensure that your bed is level when it is folded down.
Sand down the entire project with your medium- and fine-grade sand paper. Wipe it down with your tack cloth and apply your wood stain. Let the folding bed frame dry, then set your mattress inside the frame.