How you use a penis ring depends on the size and material. There are stretchy penis rings made of silicone, for example. Other materials are: TRP, TPE, leather or (stainless) steel.
The penis ring can be placed around the shaft or behind the balls. To make the use of a penis ring a bit easier you can use some lubricant.
Make sure you use water-based lubricant for safety. Some penis ring materials do not mix well with silicone or other types of lubricant.
Do you have a non-stretchy penis ring? Make sure the penis is not completely hard. First pass the penis through the penis ring and then, carefully, stuff the balls through it.