You will know the bias by holding corner to corner and gently pull a little, bias will pull, I would take a scrap piece of fabric, it will stretch. I would follow the lesson on all things frugal, someone put a link to that site on an earlier post. Just save it in your Favorites so you can refer to it as much as you need to. Once you do it exactly like she tells you, you will learn how to do it. Once you get it, you will laugh at how easy it is.
I hope this helps.