Use beads of carpentar's glue at the joining surface. Start fastening the drywall screws to secure the two pieces together. Reinforce the base with the back using three right angle brackets for extra strength. The brackets I used, came with small size screws. Attach the divider piece with the glue to the back and base piece. Fasten it with drwall screws from each side. Wipe out the access glue with a paper towel. Note that the divider in the picture below is for the bottom shelf of my design. The divider is put on the left side of the shelf. For the top shelf, move the divider to the right hand side.