Sides and bottom
Sides – Tiffany primed, painted and nailed the 1 x 4 sides in using 1 1/2 inch 18G nails with a brad nailer.
She used a square to help line it up with the window sill.
Bottom – To get the measurements for the bottom piece Tiffany measured from the outside edge of the side pieces.
She primed, painted, lined it up with the sides, leveled it and nailed it.