Take a strip of paper and wrap it around the finger you would like to wear the ring on. Mark the spot where the end has come round fully. Then lay the strip against a ruler. You now have your circumference. Bear in mind, different fingers on the same and the opposite hand may have different sizes. Therefore, measure the specific finger on which you are planning to wear the ring. Also, measure your finger at the end of the day when your finger is likely to be largest.
Make your hand as narrow as possible by touching your thumb with your little finger. Measure around the widest part of your hand with a tape measure.
Use a tape measure to measure the part of your wrist where you would like to wear it. You can also use a strip of paper and lay it next to a ruler after marking your size.