Find a flexible, measuring tape with millimeters on its ruler.
Wrap the tape snugly around the base of your finger, not too tight and not too loose. Aim for a snug but comfortable fit. Make sure to not let the measuring tape go and make sure you start from the 0 mark on the tape.
Remember the number that meets up with your starting point at 0. Note how many millimeters it took to wrap the measuring tape around your finger.
Match your finger measurement (in mm) with the corresponding ring size in the ring chart shown in method #02. If your tape is only showing in centimeter, use an online CM to MM converter to figure out your finger size in the closest millimeter.