Sew a Color 1 rectangle right sides together to the
left edge of the center pinwheel unit, stopping at a point on the
seam line 1/4" from the bottom edge of the square. Press seam
towards rectangle. Now add a Color 2 rectangle to the top, and press
that also towards the newly added rectangle. Add a Color 1
rectangle to the left in the same manner. Now add a Color 2
rectangle to the bottom edge, stopping at that same 1/4" point.
Fold the left edge of the block over, right sides together, to line up
the remaining un-sewn part of the seam and stitch...