Can someone explain/link me to a post that details how to get end game scores?

Just had this game with lucian and i got A+, not sure what i could have done better to get an s?

• Took all drakes and baron
• did not die
• loads of damage, second most on the team
• alot of gold -decent cs (179 for a 26 minutes game) -12 kills 12 assists
• almost 4 complete items, 2 more that vayne
• it was overall a stomp