criteria for an "S"

So ive been playing a whole bunch of games recently, especially trying to get an S or S+ on my favorite champs but i find that a lot of the time its really hard to do that. like Ill have a stellar game, ill go like 9-2-11 on zac for example and ill get a few dragons and maybe a baron and we will have won the game but i wont get an S for it....

Is there a criteria out there so that we know how to get an S instead of hopelessly doing good in games and having our fingers crossed for a good result