can't seem to get the lock ring to turn and lock. It is like I can't push it down enough to slip under the tabs that is locks under.

Any tips to doing this?

Maybe I have too many of the ring on it? There is a black plastic ring and a metal ring that are very similar in shape, do I need both on it? The metal ring seems to make it all too thick for the lock ring to go on. but then what is the metal ring for?