With spring time here, you might be looking for the perfect planter box plans. This plan comes from DIY network and will allow you to build a planter box of any length. The plan is well documented and can be adjusted for any window width.
Since it is the DIY network, there is accompanying video.
Good luck if you are looking to build your own planter boxes. FYI. I built my own out of western red cedar about 10 years ago and they are still holding up today.