If you know your ring size, great. If you don't and you can't get to a jeweller to be measured these ring sizers are just the thing.
Simply put the ring sizer on to the appropriate finger, it threads through like a belt.
It should be a snug fit over you knuckle and a comfortable fit on your finger. Read of the letter to the left of the arrow, this is your ring size.
They are re-usable, so please keep.