Make 2 stacks of 2 cubes each. Make sure the openings in the cubes face the same way and the outside edges of the two stacks are 48 inches apart.
Drive 1 1/2 -inch screws from the inside bottom corners of the top crates down into the bottom crates. Make sure you drive the screwheads flush to the bottom of the crates.
Place the two shelves across the two crates, so that the ends of the shelves are flush with the outside edges of the crate stacks. The centers edges of the two shelves should be tight together. Stack books or other heavy objects on either end of the shelves to hold them in position.
Drive 1 1/2 inch screws from the inside top corners of the top crates upward into the shelves.
Attach the two mending plates to the undersides of the shelves with 1/2 inch screws. The place where the two shelves come together should be about midway between the two inner holes on the mending plates.
Attach two corner braces between the inner edge of the crates and the underside of the shelves, about 1 inch in from the front and back edges of the crates. Install the other two braces on the other side in the same way.