Designed to be a snug to loose fit, you should be able to easily fit your finger between the bracelet and your skin when it is on. Check sizing chart and use a string to find your exact measurement.
Connects via a silver S hook. S hooks come locked, it must be opened slightly (about 1 mm) in order to slip over the loop. It is important to lock the S hook again so as the bracelet does not fall off. Pull the hook out (away from the hook) not to the side.