Remove all hardware from your door. Take off the hinges and the doorknob, as well as the kickplate, if your door has one. Remove locks as well.
Sand and paint the door. You don't have to remove all the old paint completely, just sand it so that it is rough and will accept paint. Paint the entire door with an enamel interior paint. This will resist stains from spills and provide a smooth surface for your desk.
Attach four flanges to the corners of the door. These flanges will have holes for you to put screws through. Use 1/4-inch screws and drive the screws through the flange holes into the wood of the door. You will see a set screw in the side of the flange that is designed to screw up against the pipe when you place it in the flange. Loosen the set screws so that the pipe will not be obstructed when you insert it.
Cut four legs from galvanized pipe. Make these legs 3 feet long. Use a hacksaw with a blade for cutting metal.
Coat one end of each galvanized pipe leg with metal adhesive. Coat at least 3 inches of the end of the legs. Insert them into the flanges and tighten the set screws. The set screws will hold the legs in place while the adhesive dries.
Slip a T-joint over each leg, starting at the leg's unattached end. Tighten the set screws in the T-joints to hold them in place.
Cut two lengths of galvanized pipe that will span the distance between two of the T-joints. These are your support braces. Apply adhesive to the ends and insert the braces into the T-joints. You should use a level placed on the brace before the adhesive dries completely, so that you know the braces are level. Tighten the set screws in the T-joints.
Attach end caps to the legs where they will be resting on the floor. Use metal adhesive on the ends of the legs to ensure that the caps will stay on.