I started making these a few years ago after I found a wonderful tutorial from Jenny at the Missouri Quilt Company on YouTube. They are simple, fast and easy to make but at the same time are durable as well.
You start with odd sizes for your front and back pieces. Generally it is 30 inches for the front side and 40 for the back. The back side is what makes the mitered corners and borders.
If you are interested in trying to make your own, have a look at Jenny's (MQC) video on how to do it.