Attach the sides to both the bottom and back piece. First, run a bead of wood glue between the boards and then nail them together. I attached three nails on each side and vertically on the back piece, and five nails across the bottom of the back. To keep the boards in place, use a wall as support
when attaching the sides. Clamps are also a great way to keep the boards secure.
If you will be mounting
your windowbox directly onto the wall, leave the front unattached so that it can be mounted easily.
And for those of you who will be using brackets
to mount the window box, go ahead and attach the front at this stage, too.