according to the instructions on the link, self binding is one layer only. I tried playing with a piece of paper to get a double fold self-binding, but while it can be done, it is really bulky (if batting is used to fill the binding, or very wimpy (if you trim the batting).
In addition, the double binding makes the corners double bulky when you follow the instructions on the link.
If you want to do it double, I may be able to figure out the numbers but you need to tell me how wide you want the finished binding to be...how much backing fabric do you want to have visible on the front.