Pre-cut the 2 inch by 4 inch lumber to get the pieces required for sawhorse assembly. The 32-inch long pieces are for the legs and are a little longer than you will need. This will allow you to adjust the desk height to compensate for differences in angles in different brands of sawhorse brackets.
Slip the 32-inch long 2 inch by 4 inch boards into the leg holes on the sawhorse brackets. Do not nail them in place yet. Clamp two leg sets onto a 2-inch-by-4-inch piece of lumber cut to the width of the door. Measure the height of the top of the sawhorse. Your goal is to have a sawhorse with a support surface 29 inches above the floor. With the thickness of the door, this should be very close to standard desk height of around 29-1/2 inches.
Remove the legs and trim them lengthwise so the supporting surface of the sawhorses is about 29 inches. Although the exact length will vary depending on the angle of the brackets, for 40-degree brackets cut 1-1/10 inches off of each leg to lower the supporting surface by 1 inch.
Re-assemble the saw horses and double-check that the height is as desired. Once verified, nail the lumber into the brackets.
Lay the door across the sawhorses.
Screw a keyboard drawer to the bottom of the desktop directly in the middle of the door.