General Store Cubbies

Build the outer box from the sides and shelves using 2" screws and glue, or pocket-hole screws.

Install the bottom row of dividers first, fastened solidly. Then install the top row of dividers — you won't be able to fasten the bottom of these into the middle shelf, since there's no access once they're standing. That's fine: glue those joints and let them set in place before moving on.

Use the trim boards to secure the bottom boards in place, nailing the trim to the shelf and sides as well. Add trim boards to the back too. Adjust for square before attaching any of this trim, especially if you're stacking or lining up multiple cubby units — an out-of-square unit will be obvious next to a second one.