Cut all the boards according to the plans using your miter box. It is very easy to use. Check out the video for more details.
Align the bottom 1″ x 4″ board to back 1″x12″ board and attach using wood glue and finish nails.
Attach the corner pieces using wood glue and finish nails.
Measure the space between dividers and attach them using wood glue and finish nails.