Determine the placement for the shelves and mark their heights. Once that's done, draw placement lines across all side pieces of the shelf at once.
Use wood glue to attach the shelves, sides, top and bottom together. Hold them temporarily in place with clamps. Then drive in nails.
Cut a piece of quarter-inch plywood for the back of the shelf, and attach it with wood glue. Use nails to permanently secure the back.
