If you aren't sure of your ring size, and you're not able to get sized at a jewellery store, this is a great way to do it at home!
Simply slide the end through the "buckle" like a belt to make a ring shape and tighten it to a comfortable level, ensuring you're still able to slip it over your knuckle! The arrow on the sizer will point to the size you are - it's as simple as that!