Nobody knows what is required for the S ranks. Don't believe people that say stuff like "get better kda", "get more cs", "ward more".
My guess, and it is just a guess, is that Riot tracks and compares how much influence you have over winning the game vs the influence that your team mates have. So if you can really carry a game hard then you'll end up with a high rank no matter what your kda and cs is. Of course in order to exert enough influence a high cs count is desirable, but only because it makes you stronger in stats due to having items and levels over the enemy. What is truly important is good decision making, are you in the right place at the right time to tip the balance of the match in your favor and thus win. The more capable you are at this the higher a rank you'll get.
But again thats just my guess.
There are other idea's floating around. Like taking your champion and going into a role you would normally never see that champion in. This is under the assumption that most people don't play the champion in this manner and thus you get compared to fewer people who are less likely to get good grades in these roles.
Ultimately we don't know though, so good luck.