Attach Side Panels to Back Panel
Use a drill and a 1/4" drill bit to create pilot holes 3/8" in from the upper back and lower back corner edges of each of the two side panels.
Note: The pilot holes will allow screws to go directly through the side panels, then into the back panel, creating a tight fit.
Add wood glue along all four edges of the back panel. Secure side panels and back panel together with trim screws drilled through each of the pilot holes.