Cut or buy all lumber to the listed dimensions.
Mark the 48-inch and 49-inch sheets of plywood. They will be hard to tell apart during construction.
Sand all faces of your lumber.
Apply one coat of your paint or wood stain to all faces of the lumber. Allow to dry.
Set one 30-by-30 plywood sheet and the 30-by-48 plywood sheet in an L-shape along their 30-inch edges. The end of the 30-by-48 sheet should rest against the inside face of the 30-by-30 sheet. Glue in place.
Reinforce the glue with a line of eight evenly spaced nails, driven into the midline of the edge of the 30-by-48 sheet.
Form a block-C shape by attaching the remaining 30-by-30 plywood sheet to the other end of the 30-by-48 sheet. Glue and nail in place the same way.
Lay the 30-by-49 sheet on top of the C-shape structure, its edges flush with the edges of the frame. Glue in place.
Reinforce the glue with a line of eight evenly spaced nails along the edge of both 30-by-30 sheets. Run a line of 12 evenly spaced nails along the edge of the 30-by-48 sheet.
Set one 48-by-6 particleboard sheet and one 6-by-6 sheet in an L shape on one 6-inch sheet of the longer particleboard. Glue in place. Reinforce the glue with two evenly spaced nails.
Form a block-C shape by attaching a second 6-by-6 sheet to the opposite side of the L shape.
Set seven more 6-by-6 particleboard sheets, evenly spaced, between the two arms of the C shape. Glue and nail them in place as you did the others.
Repeat steps one through three to build a second set of cubbies.
Set one set of cubbies on your work surface, open ends of the 6-by-6 sheets down. Apply a smear of wood glue to the open ends of the 6-by-6 sheets on the second set of cubbies. Place the second set of cubbies, open ends down, on top of the first.
Lay the 48-by-13 particleboard sheet against the back of the stacked cubbies. Glue in place. Reinforce with one nail in each corner, two evenly spaced nails along each short edge and 10 evenly spaced nails along the backs of the two long pieces of particleboard.
Putty over the heads of all nails, and any other mars in the lumber. Allow to dry.
Sand off excess putty.
Paint or stain all visible surfaces of your desk and cubbies.
Set your desk unit in position.
Place the cubbies on top of the desk, along the back edge. In most cases, you shouldn’t need to mount it more securely.