Easiest method: wrap a piece of string around the base of your finger, mark where it overlaps, then measure the string in mm. That's your circumference.
Convert to US size: 49mm = US 5, 52mm = US 6, 54mm = US 7, 57mm = US 8, 59mm = US 9.
Measure at the end of the day when fingers are largest. Avoid measuring when cold or after exercise.
Knuckle test: the ring needs to slide over the knuckle. If the knuckle is significantly larger than the base, size up by half.
Wide bands fit tighter than thin bands. For bands over 6mm wide, size up by half a size.