1) You will need a ribbon, yarn or string; a marker; and a standard ruler with cm scale.
2) Wrap ribbon, yarn or string around the base of your ring finger (or finger you wish to wear ring on.
3) Using a marker, make a mark on the string where the end meets the uncut end.
4) Using a standard ruler, measure the string using the centimeter (cm) scale on the ruler.
5) The length measured on the cm scale is directly correlated to your ring sizes. So in other words, if it measures 6.5 cm, then your ring size is 6.5, etc.
That was easy! :)