1. Wrap a string or rope around your wrist and make sure it's nice and snug.
2. With a marker, make a line directly across the strings/ropes. See next image below for example.
3. I’ve darkened the lines in a photo editing software so you can see an example in part 3 of the diagram.
4. Now straighten out your string/rope and measure from mark to mark. I drew lines on the piece of paper underneath to help show my marks. My wrist is an 8" wrist as shown below.