Remove the drawers from the desk you wish to set into the built-in compartment.
Cut off any wood that overhangs the front and sides of the desk with a handsaw.
Turn the chest on its side. Use the handsaw to trim any feet or other extensions from the bottom of the chest.
Coat the chest with a varnish or stain, if the chest is not finished. Apply the stain, which will help protect the chest from humid conditions in the attic, with a paintbrush.
Measure and mark down the length and width of the desk. Mark the dimensions on the knee wall with the pencil.
Cut along the wall outline with a drywall saw. Remove the drywall.
Use a hammer to knock out any studs that block the opening.
Cut two 2-by-4s the same depth as the chest to use as support braces. Nail them to a piece of 2-by-4 vertical blocking. Nail the two connected pieces to a horizontal 2-by-4 base to complete the braces. Set the braces against the back of the wall. Screw them into the floor and wall with a drill.
Place the chest in the opening. Push it back until the front of the chest is flush against the opening. Screw to the sides and top and bottom of the opening. Replace the drawers.