Take barrier for high engage champions so you dont get destroyed early game e.g. alistar, thresh, blitzcrank etc. and with the buffs to it, I would really recommend this.

But of they have a burst champion, such as rengar, always run exhaust to protect your adc

Early game
Early game you just need to use Q with power chords to destroy and poke their adc, use heal occasionally to help with trade-offs between adcs, try to only use the q power chord as it is the strongest for damage and their adc wont be doing too much early. DO use power chord e to escape though.
Mid game
You will want to just follow your adc, using your ult to either engage or escape. Dont be afraid to use it to escape as it could cost the lives of your teammates and you will have plenty of CDR with this build so it will be on quick cooldown.
Late game
If it gets to late game, then you will be merely peeling for your team with your ultimate and mainly using your e and your w as they provide the greatest help. With the buffs to sona's aura it will be easy to give your team the armour and movement that they need.