Depends on the type. It will be complicated to explain it just with words, but if you write in YouTube or Google “How to put a cockring” you will find a lot of videos. Don’t forget to use lubricant.
The cockrings size is their inner diameter. It is very important to choose the correct size for you, especially when you buy non flexible metal rings. Too tight cockring can cause disturbance of blood circulation in the penis. If you don’t know what your diameter is you can measure yourself using sewing flexible meter or a piece of rope. Just wrap it around the desired place where you want to put your cockring and tighten it to the desired pressure. Mark the length of the circumference. Diameter= circumference/3.13 If the mathematics is not your thing just text me the desired circumference and I will calculate it for you.
If the item allows I can engrave some short texts by hand for free. Consult with me.