S-rank requires to get minimun of 80% of the available points. (SS-rank 90%) You can follow the amount of %, if you put on the advanced HUD from the options. During the song, % will be under the points and combo meter on the left side.
So more points: bigger %. How to get good points? It's all in the technique.
Swing angle before cut – 100 degrees ideally
Swing angle after cut – 60 degrees ideally
Center cut – closer to center of arrow means earns additional points
Maximum amount of points is 115 per block. Of course there is also a combo meter, so if you make too many mistakes, 80% will be pretty hard to get.
"Do this 15 times and trophy is yours." Play songs you like, and songs you already know how to beat. I also recommend you to play those songs, wich you can beat on Expert setting, because playing on hard and swinging on like there's no tomorrow for getting more points is little bit harder than playing on hard and.. not swing like crazy.
Happy lightsabering!