Remove stair box carefully as you may want to reuse some of the pieces such as the 2x4's. Keep the top piece and set aside.

Cut drywall to where it meets wall. Add 2x4's where drywall is needed and screw into place with decking screws. (I reused old 2x4's that were already there.)

Make 2x4 frame for the 3 sides and the top of the desk. Use a level to level the 2x4's around to where you want the top of the desk (3 walls) and screw in place with decking screws.

Use the previous top piece of the stair box as a guide and cut a duplicate one to create a shelf for the desk. Paint or stain both pieces before installing. Test out the top by fitting it into place. Once the top is fit attach top with finish nails to the 2x4's. Attach the bottom with drywall screws to the 2x4's.

To make the frame around the desk shelf, use a 1x6 marking to ¾ from the top for opening about 2 inches from each end. Bottom will have to measured for location of opening. Cut opening with circular saw. Careful not to cut too far and finish cuts with hacksaw blade. Sand, paint or stain and allow to dry. Once dried install using finish nails.

Install drywall corner mold and finish all drywall. Once sanded and finished paint walls.

Cut a piece of plywood for angle bottom angling one edge for a tighter install. On the floor use 1x6 finish board and cut small bevel on edge. At the top of the angle use the 1x6 again and angle cut one edge. Sand the top, floor board and angle board. Paint or stain all boards. Once dried install with finish nails.

Cut trim to fit. Paint all trim first and install once dried.

Make holes in drywall for electrical outlet and switch. Install single outlet and double outlet box. Cut hole in ceiling for light fixture and attach light fixture box while your in the attic and then wire light fixture, switch, plugs. Use drill to drill a hole from attic into wall and run electric and internet. Install light plates.

Decorate your desk to your liking and enjoy your new space!