The first and best way
Going to a shop and get a precise measurement is preferable.
If you are in the possession of a caliper and have a ring that fits you well on the finger you desire, then another good way to measure is to get the inside diameter of said ring with the caliper. One thing to observe is that rings of softer metal, gold or silver, can and will get ovaloid and will then not give a precise measurement.
To be avoided
If you do have a ring that fits comfortably then it is possible to measure the diameter with a ruler, this is though very imprecise and should only be used in a last case senario and a great dela of care should be taken when reading off the measurement. Again an ovaloid ring will be a problem to measure as it has different diameters depending how the ring is turned. Another important thing to notice is that the measurement has to be taken at the exact middle of the ring else it will be incorrect.
No ring to measure
If you are not in posession of any ring and are not able to go to a shop, then the instructions below are for you. They will guide you throug to getting a somewhat precise measurement.
First Step
You will need a piece of paper a pair of scissors a pen or pencil and a ruler for measuring in the end
Second Step
cut a strip of paper of width 6-8 millimeters and length of 15-20 centimeter. Long enoug for you to hold it comfortably in the next step.
Third Step
Wrap the strip you cut around your finger, just below the knuckle, overlapping the end. It should fit comfertably tight on your finger.
Fourth Step
Where the strip overlaps the end make a mark for measuring. For a better result try and rewrap the strip a couple of times and see if the mark is correct.
Fifth Step
Measure the length from the start to the mark on the strip, that length will be the circumference of your finger. Remember to measure from the right end.
Last Step Step
Find your ring size from the measurement you got from the strip, again that would be the circumference.