Draw a 45-degree diagonal line on the wood, from one corner of the cut edge to the opposite corner. Cut the 1-by-4 into two equal triangular pieces, using this line as a guide.
Hold the door against the wall as you plan to use it for a headboard. If the door is extremely heavy, place it on top of two overturned paint buckets or another support. After you’ve found a height you like for your headboard, lightly mark either to the left or the right of the door where the center of the headboard’s height rests on the wall. Also make a light mark above the door to denote the center of the headboard’s length.
Remove the door from the wall area and use a long, straight edge to join both marks you made in the center of the area where you just had the door. Make a darker, easy-to-see mark in this area, and erase the other marks.
Find a stud in your wall over the bed. Starting from the dark center mark you just made, run a stud finder horizontally along the wall. When you see a flashing light or hear a beep, you’ve located a stud. Mark the location of that stud, and continue moving horizontally along the wall until you locate another, which is probably less than 2 feet away. Again, mark the location of this stud.
Measure the distance between each stud. Transfer this measurement to one of your triangular sections of wood. Hold the wood piece horizontally against the wall, over the marks you made. The angled edge should point upward and face the wall. Attach this section of wood to the wall at the studs, using 2-inch wood screws and a power drill.
Place the remaining section of wood lengthwise and horizontally across the center of the door’s back. Screw the piece to the door with its angled edge pointing downward and facing toward the door.
Hang the door onto the wall using the interlocking wood pieces you just attached to the wall and back of the door.