The ring sizer works like a belt, put the end through the buckle and push through so it sits in a ring shape. Put it onto the finger you want to measure and pull the end to tighten until it is snug around your finger, the middle knuckle is usually bigger than the base so you will need to check that you can slide the ring sizer off without loosening it. Also check that you can bend your finger with the sizer fitted. Check the number in front of the arrow and this will be your ring size.
Note be sure to measure the fingers on both hands as the dominant hand fingers tend to be slightly bigger than the non dominant hand. The temperature will also affect your ring size as our fingers slightly swell when hot and shrink when cold.