Grades are based on your percentile performance compared to all other players in the region on the same champion in the same role, and takes a lot of things into account, not just kda/cs. So stuff like warding, overall kill participation, objective participation and so on are also factors. The majority of stats tend to be per minute as well, so 230 cs in 20 minutes would be god tier korean pro level, but 230 cs in a 50 minute game is actually quite poor for example.
If you want to try to sort of 'cheese' the system to get your S's, you can play her in off-roles (so not adc) where shes not played very often and so theres probably not very many strong performances which some people find it's a bit easier (although non-meta roles are non-meta for a reason so you may have difficulty even getting reasonable performances), but otherwise it's just practice practice practice and hope for stompy matches. Just remember Riot doesn't set the requirements for grades, it's all relative, so if you don't get a grade you want, it's not riot being unreasonable, it's just that enough other players are legitimately doing it better.
Cs/min is often toted as the most important stat, but don't totally neglect warding/objectives in the process since it's easy to tunnel a bit too hard and negate the benefit of having more cs might give you.
