To make the patches you need two templates, one circular and one square. The circle can be any size you like. Use a template like a plastic lid to get even, consistent circles. (Or you can even buy templates specially made for this technique) A six-inch diameter is a good size to start with, this will give you a finished patch a little under four inches square. Figuring the square template size takes a little bit of math, but you have a computer to help you: it's the square root of the result of diameter times itself divided by two. (Don't panic! This is easier than it looks!)
1.) Get a calculator with a square root key: it looks like a checkmark. (Use your computer's calculator function)
2.) Multiply your diameter (minus seam allowances) by itself.
3.) Divide that number by 2.
4.) Hit the square root key.
5.) The number you have is the length of one side of the square template you will need. It should be, roughly, two-thirds of your circle diameter.
If you are working with a template such as a plastic lid, and you don't know the diameter, or you really hate math, you can do this: trace your template onto paper and mark the seam allowances. Cut out along the seam line. Fold into quarters. Unfold and use a ruler to measure the distance between fold lines along one edge.