Measurement is key and I prefer the steel myself. Use a piece of cord to measure around the base of your balls and cock and then match that diameter to the ring. I warm it in my hands, then, as someone posted previously, slide my cock through followed by each ball. I get everything adjusted quick and get hard and stay harder longer. The leather and rubber models tend to pinch and bind and the steel is easy to keep clean by comparison. Plus, steel matches my guns and boots!
Cock rings (the rubber and steel ones) are put on whilst flaccid and then you get hard.
The rubber ones are great (like a big O-ring).
First you carefully put your balls through one at a time and then lastly slide your flaccid cock under the ring. A bit of lube on your cock helps.
When it has been on for a bit long...your cock is very dark as are your balls. If you've just noticed it, don't freak out. Just relax...do something non sexual and as your cock deflates a bit, you'll have enough room to get it off.
Taking it off: reverse the "putting it on" procedure.
I used to have a pic of a guy wearing one in another thread, but my hard drive crashed a few weeks ago, and I lost everything. If I happen to find the pic I'll post it for you to see it for yourself.