Posterize?

Howdy.

I am looking to make a lens, of course, and I'm having an extreme amount of difficulty with the software as a whole, specifically the material editor?

I wish to apply a posterization effect to my lens, just to add a few touches to it. However, it seems to be extremely difficult to even apply the simplest of effects which is frustrating. 

There is 0 documentation / tutorials that I could use to apply to my needs, which also makes it difficult. So, TL;DR, how do I apply a simple posterization effect?

Thanks.