For the inner dividers, start by cutting the middle length piece (aka the spine). This should be cut to the same length as you did for your bottom. Mark the center of the box on either side piece so you can easily line it up when affixing. Once cut, secure into the box using glue and a couple of nails through the bottom and sides. Now, you need to cut 4 pieces the same size (providing you have all of your measurements correct and the center piece is in the center!).