Drill the necessary holes. Before mounting the vertical sections in the box, either drill holes for the supporting pins or cut grooves (dados) to mount the shelves into. Measure the pin diameter with calipers and find the right bit for a somewhat loose fit, or do trial and error on a scrap of wood. Make sure to drill the holes perpendicular to the wood- use a drill press if you have one. A template to mark the holes is very helpful; If you're not tool-handy skip this step and you can use L-brackets to hold up the shelves later. It's ugly, but functional