How do you get S ranks?

How do you get an S rank? I've gone 12/1/8 with 120 cs, but never got an S rank. But the next game, a kindred with 12/8 got an S+. I then went 13/8/5 with 230 cs... and I got a D. How does the ranking system work? I did better than the Kindred both games, but i got a worse score. Please someone tell me the guidelines of an S rank so I can get one for the new mastery 7. And if a Rioter finds this, please explain the rules of it, or (Not to sound rude) Fix the broken system. Please and Thank you all for taking the time to read this.