http://www.quiltuniversity.com/squaring_up.htm
This short video shows cutting actual fabric, and is easier to understand than computer generated graphs. Those graphs are so helpful most of the time, but on this one, we want to see the uneven edges that need to be trimmed off.
Also, you don't really need special rulers. People that have them say they are worth the money, but it is possible to do this with any rotary ruler. The big ones would be very clumsy, but it is doable.