This is going to sound dumb af, but its EZ mode for an S. Get a good vision score, avoid dying and farm. Ignore your team and get a good farm going. Rito's formula is so heavy on farm it is dumb. I get it for ADC, mids, or top, but JG and support should not be so dependent on CS. When I play support or JG, I can get an S so many times, but by getting high CS.
Typically I try to make sure to lay vision, be near teamfights, and die little, but do some wave clearing especially late in game when you have a lane that is pushing. When you go 10/1/15 as a jungler and win solidly and only get an 'A' with a farm of 100, but then go 3/2/8 and get an S+ with 250+ cs even though your team lost because you were farming when you should have been helping with an objective or in a team fight. I also try to make it a point to last hit turrets when I am going for an S.
So in reality don't worry about your team, and just go for the raw numbers. Good grade, does not mean a good team player.
Edit> Someone told me this and I did not believe it at first, but then once I tried it, it was annoyingly easy to get an S.