Lay a piece of 3/4-inch plywood measuring 12 by 16 1/2 inches flat on your work table. Measure up 5 inches on one of the 16 1/2-inch sides and mark a 6-inch-long line that goes perpendicular to the 16 1/2-inch side. Measure up 3/4 inch from the first line and mark a second, parallel 6-inch-long line.
Measure up 5 inches from the two lines, and mark in another pair of parallel 6-inch-long lines that are perpendicular to the 16 1/2-inch side. You should have two pairs of lines that divide the 16 1/2-inch side into three equal portions of 5 inches. Repeat on three more pieces of 12-by-16 1/2-inch plywood.
Cut out between the lines with a jigsaw, so that on each piece of plywood you have two 6-inch-long and 3/4-inch-wide slots.
Paint the slotted pieces of plywood, as well as two more pieces of 12-by-16 1/2-inch plywood, and two pieces of 12-by-18-inch plywood. Leave the pieces to dry before continuing.
Apply wood glue to the slots on the plywood. Stand two of the plywood pieces 5 inches apart with the slots facing upward, then fit the other two slotted pieces of plywood across these two, slots facing downward, so that the slots mate. The point where each pair of slots mate will form a cross, creating nine square cubbies.
Glue the two unslotted 12-by-16 1/2-inch pieces of plywood over opposite sides of the mated slotted plywood, then glue the 12-by-18-inch pieces over the top and bottom to form a frame around the cubbies created by the slotted plywood pieces.
Drill three pilot holes through the sides of the top and bottom panels into the ends of the top and side pieces of plywood, then secure together with 1-inch screws through the pilot holes.
Cover the screw holes with wood putty and scrape the putty flat with a putty knife before leaving to dry. Retouch the puttied areas with paint to finish your square box wine rack.