A - WHEN YOU KNOW YOUR SIZE
When you have your ring size or finger measurement (in millimeters), you could put your information on any online ring conversion website LIKE THIS
B - USE A PRINTED SIZE CHART TO MEASURE YOUR FINGER
You can download the size chart HERE
C - WITH A STRING / OR STRAP
1 - Wrap the string snug around your finger, make sure it's nice and tight and cut it carefully;
2 - Measure the distance with a ruler (in mm); that's your finger's circumference;
3 - Use the CHART to find your ring size and voila, you're all set.