Put all four pieces of the 12-by-36-by-½-inch Baltic plywood on a worktable. Hook the end of a measuring tape on one end and make marks at 9, 9½, 18, 18½, 27 and 27½ inches.
Set the framing square on the marks and draw faint lines across the 12-inch depth of the plywood. Do this on all four pieces. These are the sides to two of the display shelves.
Apply wood glue on opposite edges of four of the 14 pieces of 12-by-12-by-½-inch Baltic plywood. Place one at the top and bottom of the sidepieces and secure them with 3-penny finish nails. Clean up the excess glue immediately with a damp cloth.
Next, apply glue to the opposite edges of six pieces of the 12-by-12-by-½-inch Baltic plywood. Put these pieces between the lines on the sidepieces and secure them with 3-penny finish nails. Clean up the excess glue immediately with a damp cloth.
Build a box with the remaining pieces of 12-by-12-by-½-inch Baltic plywood. Put some glue on the edges of the two pieces that secure between the other two pieces. Secure them with 3-penny finish nails. Clean up excess glue immediately with a soft cloth.
Stand the two 36-inch-tall display shelves upright. Secure the 12-by-12½-inch box between them with ¾-inch wood screws. The box can be secured anywhere between the two 36-inch-tall display shelves.