There are lots of youtube videos, here's one:
https://www.youtube.com/watch?v=KNxICsLAOrE
There have been several posts on this board about making your own ruler, if you do a search you should be able to find them. They provide the angle.
I have the actual tool. Compared with other tools, it's pretty inexpensive, also, because you are placing it over intersecting seams on your starting piece the feet raise it up a bit and keep it from rocking. This makes it a bit safer & more accurate (for me, anyway).
A couple of comments. There is a fair amount of waste. Don't cut too far ahead of yourself, it's easy to get the pieces mixed up. I found a large rotating mat was very helpful.