Use a ring sizer or ring gauge
Place a ring that fits well on the sizer
Match the inside edge of the ring to the correct size
Measure the inside diameter with a ruler if needed
Wrap a strip of paper or string around the finger
Mark where the ends meet
Measure the length with a ruler
Compare the measurement to a ring size chart
Measure the finger at the end of the day
Measure more than once for accuracy
Ensure the ring can slide over the knuckle comfortably