I use a cloth measuring tape. But you can also use a strip of paper 1" wide. Mark it where they overlap and use the ruler to get measurement number. Much easier than a string in some ways.
don't use thick string or nothing, it'll mess up your measuring, use the thinnest thing you can find, like a piece of paper. better yet, make aa ruler out of a piece of paper
Use antything ya want.
String, tape measure, paper even WORMS.
If I was far away from it all [world] and wanted to measure ma girth, had no string etc....than a worm would do lol or a rats tails hahaha