When you are starting your game, charge your Powercord in base 2 times.
That way when you get into lane phase you can early burst enemy players instantly with the Q + Auto Attack combo.

Throughout the game, you will always want to try and have your 2 Powercord stacks ready for the surprise burst.

If you do happen to get the Dark Seal above 8+ stacks and you know you are dominating, buy a Mejai's over the Ardent Censer as your 4th item. A 25 stack Mejai's and Powercord burst can potentially 1 shot.

Also remember that Sona's Q range is huge. Don't be afraid to harass your opponent with it when your Powercord is not ready. Nothing is more annoying to an opponent than when they are just trying to farm and all the sudden their health gets deleted.

Warding is very important for both you and your teammates.
In your early game, use your wards on the bottom brushes so you can always see your opponent. That way you can harass with your Qs and force them away from the minions to lose EXP.
Nothing hurts more than when you press Q and it doesn't hit the enemy because they went into an unwarded brush.

On your first back, if you have enough gold, buy a control ward to place in the upper brush so you don't get ganked.
Make sure before you place a control ward your lane is pushed out at least to the middle. There is no point of putting a control ward on the upper brush if you are hugging the turret because then its just free gold for the enemy and you wasted a control ward. At that point just put a regular ward instead.

Don't be afraid to accidentally Kill Steal. A secured kill is way better than no kill at all.
The point of you playing Sona with this build is to make sure no one gets out alive.
Especially if you know your ADC is just really bad and can't last hit even if his life depended on it.

Pros:
Squishys have no chance against you.
Nami and Soraka can get in the way with the heals but it is no threat since you will most likely damage more than they can heal.
Even Morgana's Black Shield can't fully stop a Q + Powercord Burst.
Your ADC might cry because you "Kill Secured"

Cons:
Braum's E can potentially stop you when trying to Burst Q + Auto Attacks.
Yasuo wind walls are also bummers so play smart and move around them.
Your ADC might cry because you "Kill Secured"