I've achieved various Ss (S+, S and S-) on Sona this season. So, take this advice as you see fit...
General advice:
Limit your dying (usually 2 or less). Get lots of kills and/or assists. Ward alot (generally) and ward sweep (if on the Rift). Make sure that relative to your team you are in the top 3 players. As for the types of wards, pink wards do count more - but barely.
Special note:
Some supports can be played aggressively in some situations - Alistair, Karma and Zyra for example -- even Sona. So, don't be surprised if a decent kda is not enough if your kills are low.