Assemble a box out of 2-by-12-inch boards, two of them 40 inches long and the other two 79 inches. The shorter boards should be sandwiched between the longer ones. Clamp the box together with bar clamps.
Drill four 1/16-inch pilot holes in each corner joint, and secure the box together with 3-inch wood screws through the pilot holes.
Clamp two 75-inch pieces of 2-by-2-inch board along the inside of the long boards, 4 inches up from the ground. These will form a support for the boards holding the mattress.
Screw the 2-by-2-inch boards to the 2-by-12-inch boards with 2 1/2-inch wood screws, drilling pilot holes every 3 inches first to prevent the wood from splitting.
Set 15 40-inch pieces of 2-by-4-inch board across the box, resting on the supports you have screwed in place. Space them evenly.
Drill two pilot holes through the outer wall of the box and into the end of each 2-by-4-inch board. Screw the boards in place with 3-inch wood screws.
Flip the bed frame upside-down.
Stand a 65-inch 4-by-4-inch post in each corner of the bed frame, and clamp them in place.
Drill six pilot holes per corner, with three on one side of the corner and three on the other. Screw the bed frame to the posts with 4-inch wood screws.
Flip the loft bed rightside-up and stand it on its legs.
Clamp a 76-inch 2-by-4-inch board to the rear legs, positioned 4 inches above the ground. Drill pilot holes and screw it in place with three 3-inch wood screws per side.
Clamp two 38-inch 2-by-4-inch boards across the posts on the ends, positioned just above the board from the previous step. Screw them in place with 3-inch wood screws. These three boards will support the legs and prevent them from moving.
Cut a 75-by-24-inch piece of 3/4-inch plywood.
Drill a 1-inch hole in each corner of the plywood, positioned 2 inches in from the corner.
Thread a 10-foot chain through one of the holes, under the plywood and up through the other hole along the 24-inch edge. Thread a second piece of chain through the other two holes.
Loop the chain around two of the 2-by-4-inch boards stretching across the bed frame opening. Secure the loops together with threaded links to create a hanging desk.
Further secure the desk to the bed frame legs with zip ties if more stability is required. The zip ties can be threaded through the drilled holes in the plywood, and tightened around the legs.
Add a computer and whatever other work equipment you need to complete the work station.