Apply a paint stripper to the surface of the old door using a paintbrush. Let the stripper work for 15 to 20 minutes. Once the stripper wrinkles, rub the surface with a circular motion using a steel wool pad. Repeat the process on all sides of the door.
Apply lacquer thinner to the surface of the door using a paintbrush. The thinner pulls up any remaining paint and stripper on the door's surface, which would make it difficult to repaint it. Follow the thinner’s directions for proper application. Allow the door to dry for 24 hours.
Paint or stain the door the color you want for the headboard. Let the paint or stain dry overnight.
Locate a stud on the wall where you want to hang the door headboard using a stud finder. Measure up 28 inches from the ground. Attach a flush mount wall bracket to the stud using screws. Measure up 60 inches above the ground. Attach another wall bracket. Locate four more studs over the 80-inch width where you want the headboard. Attach two more wall brackets to each stud at the same height as the first two.
Place the door face down on your work surface. Turn the door so one of the 80-inch edges is in front of you and the 36-inch edges are on your right and left. Attach adjustable flush mount brackets to the back of the door across the 80-inch side. Place them all 2 inches down from the top edge and spaced the same spacing as the brackets on the wall. Screw the screws through the holes in the bracket into the door. Lift the door up and slide the door brackets onto the wall brackets.
Cut two pieces of 12-by-12-by 1 1/8-inch plywood using a circular saw. Sand the cut edges with medium-grit sand paper. Paint or stain the plywood pieces to match the door. Let the plywood dry overnight
Place the first plywood piece face down on your work surface. Center an L-bracket on one edge of the plywood. Attach the bracket to the plywood using screws. Repeat the process with the other piece of plywood using another L-bracket and more screws.
Hold the first plywood piece up to the wall-mounted door headboard. Align the edge of the plywood with the edge of the door. Position the bracket pointing down and attach it to the door using more screws. Repeat the process on the opposite side of the door with the other plywood piece and more screws to create two plywood nightstands.
Place a full-sized metal bed frame between the space of the two plywood nightstands. Place a box spring and mattress on top of the bed frame.