What are the requirements to get S- or higher?

I main a few select champions and I play some others just for the S- and the chest. But I want to know is what are the requirements to get at least S because I'm pretty sure everyone hates seeing A+ on any champion. Here is the match - http://matchhistory.euw.leagueoflegends.com/en/#match-details/EUW1/3397234545/229145328?tab=overview I know I died quite frequently and those could have been avoidable like a heal/flash but sometimes I get more deaths and I still somehow get S- on other champions. Also, I got the most CS, Kills, highest vision score and damage.