The assembly of the sides is very straight forward.
Using #6 wood screws, the top and bottom act as lintels, overlapping the side pieces to form an internal square exactly 4" x 6".
Working one corner at a time, mark the holes with a pen or scribe. We'll be using two screws per corner, to prevent any sides from twisting out over time.
It's important to aim more towards the middle of the edge, and drill the pilot holes (using 1/8" bit) very slowly to prevent the plywood from cracking.
Once the holes are drilled, slowly screw in the wood screws, and move to the next corner.