Measure and cut the plywood into two 12 1/2-by-24-inch panels, two 12 1/2-inch squares and one 14-by-24-inch rectangle for the back of the shelf.
Stand one 12 1/2-inch panel on end. Set the 24-inch panel on top, butting the 12 1/2-inch ends together. Using the carpenter's triangle, check to ensure the pieces are square.
Predrill three screw holes through the 24-inch panel and into the end of the 12 1/2-inch panel. Apply construction glue and screw together. Repeat at the opposite end of the 24-inch panel.
Turn the shelf over. Fit the second 24-inch panel on top, again butting the ends over the 12 1/2-inch ends. Predrill, apply construction glue and screw together.
Turn the shelf onto its face. Lay the last panel on the back of the box shelf, aligning all edges. Predrill the screw holes, 3 inches apart, along all sides. Apply a bead of construction glue to the edges of the box. Screw together. Allow the glue to dry overnight.
Fill any spaces along the edges of the plywood and cover the screw heads with wood filler. Use a putty knife to smooth the wood filler. Allow to dry completely according to the manufacturer's directions.
Sand all sides and edges of the box shelf smooth, using a fine grit sandpaper. Wipe with a tack cloth to remove all dust.
Brush a coat of primer paint over the box shelf, inside and out. Allow to dry.
Apply a coat of latex enamel paint to the box shelf, using a paint brush. Allow to dry and apply a second coat.
Brush a coat of polyurethane over the entire box shelf. Allow to dry completely. Sometimes latex enamel stays sticky, even after the drying time is complete. By allowing to dry according to the manufacturer's directions and then applying a coat of polyurethane over the paint, you avoid damaging the cardboard covers of your albums.