Measure the area where you want to put the desk. The desk will be placed against a wall, so measure there. A typical door is sized between 30 to 36 inches wide and 80 inches long.
Sit in the chair you will use at the desk. Measure from the floor to a comfortable desk height, and mark that height on the wall. An average height is between 26 and 30 inches.
Stretch out a chalk line along the wall at the height you want the desktop. Use a level to check that the chalk line is straight and level. Pop the line to make a chalk line across the wall.
Remove any hardware -- like a door knob, deadbolt or hinges -- from the door using a screwdriver.
Find three to five studs along the length of the chalk line and mark their locations with a pencil.
Attach the brackets to the wall using two 2-inch drywall screws into each bracket and securing them into studs. Screw them into the wall with the top of the bracket 2 inches below the chalk line.
Place the door on the brackets flush against the wall.
Screw through the brackets into the door with the drywall screws.
Paint or stain the desktop and seal it with three coats of clear polyurethane.