Take a strip of paper that is 3mm wide (width is crucial) and 100mm long.
Wrap the strip of paper around the base of your finger. Make sure it's snug, but not too tight. Otherwise you could get the wrong size.
Now glue the joint with adhesive and try putting it on and taking it off several times. Start over if you feel it's necessary. You should be able to put it on and take it off without any problem.
Cut the paper in half, measure the length, and compare it to the table below. The measurement that you must take into account is the measurement of the circumference of your finger (mm).