This build is all about Maximizing Mana/Mana Regen, and CDR.
With these items, you have enough mana and mana regen mid-game that you can keep your whole team sped up, and healed/shielded for insane amounts of time before running out of mana.

Sona is all about Auras. So make sure to get close to whoever you want to heal so they get shielded too. Make sure to use your E to speed up your team constantly.

This build is all about using her Passive Stacks correctly.

Upon reaching 3 stacks, Sona's auto-attack resets and it allows for an instant super-powered Auto-attack.

How to properly Poke: Get 2 stacks, then click A + on an enemy champion, once your basic attack goes off, hit q, and it will automatically supercharged auto-attack again. It is important not to right click until the combo fully went off, if you right click after the Q, you can accidentally cancel the supercharged attack before it goes off.

How to properly use E: Use E all the time to speed up your enemies. But, when chasing down an enemy champion, or when running from a champion, for example, getting ganked. Ready up 2 stacks with q and w, then use E, then auto again to slow them down. This is a great way to escape, or start an initiate on an enemy champion.

How to properly use W: You can use W after getting 2 stacks on champions like Yasuo when he ults and Katarina, as it reduces their damage output. Other than that don't worry about using supercharged W. The main thing is to use W to heal and shield allied players. Remember the shield doesn't work on allies unless you are near them. So sometimes it is worth it to flash onto an allied champion and w to save them.

How to use R properly: When getting ganked. Either R instantly on the jungle or adc. or wait until the perfect moment to land R on all 3 of them.
Another great way to use R is to initiate with Flash and R. Wait until the enemy team groups up, and make sure your team is nearby. then Flash ult to hit as many of them as possibly.