Getting an accurate measurement comes down to timing, technique, and a few details that are easy to overlook. These tips will help you get the most comfortable and reliable fit.

Measure the hand you'll wear the ring on. Your dominant hand is generally slightly larger. Even a small size difference between hands can affect fit, so always measure the specific hand and finger where the ring will be worn. 
Factor in band width. Wider bands fit more tightly than thinner ones. Plan to size up by at least a half size for chunky bands (over 4mm) and size down accordingly for very thin designs (under 1.3mm). Ring sizes are measured in millimeters (mm), not inches. 
Account for temperature. Fingers shrink when cold and expand when warm. Measure when your hands are at a normal, comfortable temperature to avoid an inaccurate reading. 
Measure at the right time of day. Fingers naturally fluctuate in size throughout the day based on activity, hydration, and diet. The most reliable time to measure is toward the middle or end of the day, when your fingers are at their largest. 
Test for knuckle fit. If your knuckle is noticeably larger than the base of your finger, measure both areas and choose a size in between. The ring needs to clear the knuckle comfortably without sitting too loose once it's in place. 
Measure more than once. Finger size can shift slightly from day to day. Taking two or three measurements at different times and comparing results gives you a more accurate average. 
When in doubt, size up. If you fall between two sizes, go with the larger one. Rings can generally be resized down more easily than up.