This is my custom build for Sona, This build is for more careful players and doesn't really focus on damage.

Some things to remember:

Always follow someone else around, and never go by yourself, because you are an easy gank.

Never take a Solo lane because you will lack the means to defend it properly, try to last hit as much as possible because otherwise you won't be getting much gold and with this build you want to get to the point where you get your boots ASAP, until then play extra careful and try to never over extend lanes, and ward the sides so you see gankers coming.

Song of Clarity+Shurelya=TEAM SPEED BOOST!

Always watch your passive, the damage reduction or slow can really save people.

Always sit in the back during team fights, You can heal from a great range and if you see an opportunity to use your ultimate, do so and immediately retreat behind your team mates.

You will get stacks for your Mejai's from assists, It will be your only source of AP for a while after getting it so do whatever you can to get assists, you can kill targets too, but don't try to KS from someone who needs it if they can get the kill such as a carry.

If your lane partner leaves you all alone, ***** until they return or someone comes to help you.

When the game starts put your first point in Hymn of Valor, and sit in your base and use it two times so the next time you cast it your power chord is up, pick up a regrowth pendant and go to your lane.

Try to last hit as much as possible, once you reach 365 gold go back to your base ASAP and buy your Philosopher's stone.

When you reach around 1000 gold go back and buy your tear, try to use spells whenever you can in order to fill the passive mana boost quickly, but don't oom yourself. (Spam spells on base trips or if you have blue buff)

I normally didn't buy boots until after I finished my Mejais, but it gets far to dangerous being super slow and the passive speed from celerity won't be enough, but with a normal pair of boots it should put you a little slower than most champions.

I finish my speedy boots ~20 minutes in a match.

Spam song of clarity when moving around but try to save the speed boost from Shurelya in case someone is trying to escape from something or if you're in a hurry, it also affects team mates so it could be a nice sign for a team fight initiator. (Rammus also loves it when he is rolling around.)

Late game you should be able to spam song of clarity and your heal almost indefinitely and the CDR from this build puts them both on a ~3 second cooldown.

Use your hourglass if you get focused and immediately stun and flash away for an escape. (Using the stun and flash is also a good way to escape any situation, but I have trouble running from Yi's or Tryndameres some times so always keep extra distance from them.

I plan to write more soon.