Go to Alicia's Attic on line and they have a few free videos you can watch on binding odd angles - Marci Baker is the demonstrator, I think. Paste this address in your browser, as I am unsure how to provide the link on this page. There is one specifically on the shape you want, and it is on You Tube as well.
http://www.online-quilting.com/video...ree/index.html
P.S. I have just noticed that the post above mine is the same video - with the You Tube link. It is an excellent tutorial.