Measure the walls from corner out to determine the depth of the cupboard. Then, measure the height and record all the measurements.
Draw results from Step 1 on the ½ inch plywood and cut them out. Miter the front edge of the side panels at a 45-degree angle.
Determine the number of shelves you want for your corner cupboard. Draw them on the ¾ inch plywood and cut them out.
Space the shelves as desired and secure them to the side panels with the 1 ¼ inch screws. Measure and cut two 1-by-2 inch solid wood pieces for the height of the frame and miter one edge on a 45-degree angle.
Place the shelves on the cupboard to ensure a good fit. Measure the space between them and cut two pieces of the 1-by-2 inch wood for the top and bottom of the cupboard frame.
Glue the 1-by-2 inch frame pieces to the cupboard and secure them in place with the 4d finish nails. Set the nails with a nail set. Let the glue set up over night.
Secure the corner cupboard to the wall with 2 ½ inch screws. Use a level to plumb the cupboard to the wall.