If you already have a ring that fits you perfectly, you can measure the diameter and find the corresponding size in the chart above. Just measure straight across the inside middle of the ring. Take a few measurements for confirmation.
Wrap a piece of paper around the intended finger and mark where the paper overlaps. The length of this paper is the circumference of your finger, and the corresponding ring size can be found in the chart above. If the size of your knuckle is drastically different, take a measurement of that as well, and go with something in the middle of those two measurements.