As ADC
Get 10+ kills. Try not to die more than 3 times. Farm till 200-300 minions.
Even if your team loses, and aslong as you don't die more than a good 3-4 times. you have an Guaranteed s+
With every role, high CS is what gives you an S. I just tend to roam to steal kills, buy a runnans, afk farm and wait for the game to end.
That's how to get an S :)
The idea is to match your performance towards the top end players. High CS is the dealbreaker because once people start getting fed, they stop CS'ing.
So basically,. had you gotten another 70+ cs, you would have scored an S
http://i803.photobucket.com/albums/yy318/joetri10/ryttrg_zpsoex2aeuh.png
Here's an example as a jungle.
Note: On the S game, the game was not only 7 minutes longer, 4 kills less and 2 more deaths, the CS roughly 50 higher, and that is what made the difference between the S- and S.
Had i had the same CS on the S- game, it would more than likely had been an S+ game.