Cut and Mark the Cleats
Since the piece is angled, the sides and cleats need to be mitered at the front so the front facing will lay flush. Set the circular saw at a 45-degree angle and cut the plywood sides using a guide to keep the saw straight. Cut the cleats to size, being sure to miter the front ends.
Lay the sides next to each other on a flat surface. Use a large square to draw lines across both boards to mark where the 1" cleats will hold up the shelves. Run a bead of glue along the cleats and nail each in place. Assemble the two sides together using a butt joint, wood glue and nails.