Is it best to put it on when your still soft or after you have an erection?
It might depend on the kind of cock ring you have. For instance, metallic cock ring (as found in Gates of hells, but not only) are impossible to put with an hard erection.
In fact the challenge with that kind is more to remove it than put it on. Just relax... and get me the knife

Leather or plastic cock ring are adjustable (somehow), so might be put on already erected material. I believe it's a matter of personal taste.
And I'm assuming it should be fastened as tightly as possible. I have one that I purchased recently but haven't used it yet. I'm anxious to try it out.
Not too tigh, you still want the blood to :
- come in
- refresh (partially come out), unless you're on CBT (but that's another story).