These thin rubber rings are available in 4 different sizes, and can either be used as cock rings or as removable harness rings for smaller or bigger dildos.
Non-adjustable rubber rings don't make the best beginner cock rings, as they're not very flexible and may be difficult for beginner users to remove.
Using cock rings can make the penis harder for longer by slowing the blood flowing back down the penis. A cock ring should provide increased pleasure to the wearer, so if it doesn't feel good, take it off. Also, if you're having trouble losing your hard-on while wearing a cock ring, you can try running your penis under cold water.
To determine which size cock ring to wear, first measure the circumference of the penis behind the balls (you can use a piece of string to make this easier). Because cock rings are normally sized according to diameter, divide the circumference by 3,14 to get the diameter measurement. Make sure the ring size is no smaller than your penis diameter.