The best and most accurate way is to get a reusable ring sizer. Can get one here for free. (click the image)
Or alternatively you can do it at home
Paper or String
    1. Start by cutting your string or paper to 120 mm long.
    2. Wrap the piece of paper around your chosen finger as tight as you can, try not to rip it! If using string make sure it is nice and firm. 
    3. Mark the spot the two pieces overlap. The length of paper or string you have measured in mm is the circumference of the intended finger. 
    4. After finding this measurement use the chart below to find your UK size. 
    Please note:
    I made this size chart as the best way to get your ring size at home. This chart has been altered slightly for this purpose. 
    Not to worry, if you do get it slightly wrong you can send it back and I can resize it!
    First time free of charge any times after that there's a small fee of £25