Cock rings (also known as C-Rings) are sex toys that constrict blood flow and keep it from leaving the penis to produce heightened sensation and maintain firmer, longer-lasting (in most cases) erection. Cock rings are worn around the penis, either at the base of the shaft or around the base of the shaft and behind the testicles depending on the amount of constriction you want. They can be worn during both solo and partnered play.
If you intend to place the cock ring around your shaft and testicles, your penis should be semi-flaccid and flexible enough to bend. Start by stretching the cock ring and putting one testicle in at a time, then push the shaft down through the ring before bringing it over and around until it is lying flat against your pelvis. Use lube to cut down on friction as you adjust the cock ring into place.
Once you're familiar with putting on a cock ring, you may find that you may need more constriction after a while. This is where medium-stretch cock rings come in. These types of cock rings still have a lot of elasticity but are more solid and offer more constriction of blood flow.