amma, don't we have a tutorial on this. I remember, somewhere in my mind, seeing photos. ???? I will look in search but you have a photographic memory and I don't mind if you find it before I do. :)
On Edit:
This one is good (and has a video):
http://www.quiltuniversity.com/squaring_up.htm
I found the reference in
http://www.quiltingboard.com/t-39236-1.htm