Hello Sawsan,
You can use this ruler for many different shapes and angles by lining up the the marks on the ruler with the lines on your cutting board or the shape of your fabric.
I'm sending a link to a 'Google' search page with many tutorials and videos for ruler use. Although I did not find an example of your particular ruler, they all work in the same way. Hope you will find these helpful. I looked at several of them and I liked the helpful tips.
Here's the link:
http://www.google.ca/search?q=quilt+...ls=org.mozilla:en-GB:official&client=firefox-a
I can't think of a good way to explain without pictures!