Don't know your ring size?
Simply pop into your local jeweller (or me) to get sized accurately.
That is it, there is no other option.
DON'T use one of those horrid plastic ring gauges, a printed template or any other form of DIY ring sizing.
Plastic ring gauges are just rubbish plain and simple. If a company sends you one then they are more interested in trying to get your sale than they are the best fit. Their interest is self serving. No serious jeweller would consider these.
Ring diameter's or ring circumferences are made to an accuracy of 1/10th of a mm. You just can't measure that kind of accuracy at home with a ruler. Please don't try.
Your finger is not a single ring size
You might be a size "M" for a ring that is 2mm wide but for a ring that is 10mm wide, you are probably a ring size "P"
If you are after a 2mm wide ring, get measured with 2mm wide rings. If you are after a 10mm wide ring, get measured with a 10mm wide ring.
You can't get an accurate ring size using anything that is flexible (like those horrid plastic ring gauges or a piece of string)
If you have a ring that you know, fits you well, please take that to a jeweller and ask them to measure it.
Don't get measured for a ring if the jeweller gives you one of these "Jailor's Keyring" type of ring sizers. The sheer weight of the whole contraption will not give you an accurate ring size. If a store gives you one of these, they really shouldn't be selling jewellery.
So to summarise
Option ONE : Visit a local jeweller or me
Option TWO : There is no option two.
Hope that helps