Use a ring sizer tool or printable ring size chart
Measure the inside diameter of a ring that already fits you
Wrap a strip of paper or string around your finger and mark the overlap
Measure the marked length with a ruler
Check your size at the end of the day when your fingers are largest
Measure the exact finger you will wear the ring on
Make sure the ring can slide over your knuckle comfortably
Compare your measurement to a ring size conversion chart
If between sizes, choose the larger size
Have a jeweler measure your finger professionally