This guide is a work in progress, please tell me your experiences with this build so I can make it better!

I'll include important combos at their stage in the game, these will primarily exclude your Ult, as you will usually use these without it. (Just insert ult where you feel necessary in any combo)

At this time you will be extremely weak, at least compared to most top laners. This is where it's most important to try and keep the wave just outside your tower as much as possible. By doing this, you will be able to prevent early deaths, and are able to harass the enemy laner more freely. Don't be discouraged if you're taking heavy damage, just hang back and keep poking with Q and your passive A. You'll be surprised how easily you can turn a 1v1 once you hit 6 if you stand your ground and keep harassing. (And trust me when I say the more you tempt the enemy with your low health, the more likely you are to end up with a kill with tower)

Important Combo(Trading): W Aria of Perseverance -> E Song of Celerity -> Q Hymn of Valor -> AA

Here you will be able to do a bit more, at this point you will have your ult, and with it's passive you can increase your safety to push and poke. You will also want to consider looking around the map for places to teleport or gank. Applying pressure to your lane isn't as effective as Sona isn't as capable of pushing a wave or taking towers, helping another lane then pushing yours with their help (or the jungler's) with benefit you and your team way more.

Important Combo (Protect): W Aria of Perseverance -> Q Hymn of Valor -> E Song of Celerity -> AA

Important Combo (Protect #2): E Song of Celerity -> Q Hymn of Valor -> W Aria of Perseverance -> AA

Your primary focus in a team fight will be similar to a support (cause ya know, Sona's a support) protect your team as much as possible, and ensure you block as much damage as you can.
I tend to stay right in the middle of my team where I can give everybody my auras, and quickly shift focus onto protecting a specific carry. When using Redemption [Redemption] try to get as many enemies and allies within it, preferably while your team is around 50% health, as it will give you a surprise advantage on the enemy, using redemption + R Crescendo you can buy time for your team in a losing fight, or strengthen your advantage. Immediately after a Redemption + Crescendo combo, you should use your W Aria of Perseverance to boost their health further, following up immediately with Q Hymn of Valor for additional damage while the enemy is open. Be sure to aa after each spell if you have any spellblade items, if not make sure you aa every 3 spells, this way you will maximize your damage output.

Escaping a Losing Fight:

If you're not low ( < 40% Health), stay behind your team and deny as much damage from the enemy as you can.

Focus on using your E

Song of Celerity to get everyone out quickly and W Aria of Perseverance to deny damage and keep your team healthy. Still use your Q Hymn of Valor when you can to provide damage, in case you gain a chance to turn the fight.

Wait for enemies to use key abilities and/or movement abilities before using your ult, this will make sure they can't catch up as easily after they've been ulted (also makes it easier to land).

Important Combo (The Gam Special): Flash Flash (Only if the enemy team is on top of you / too much damage or CC, etc) -> Redemption Redemption -> R Crescendo -> W Aria of Perseverance -> AA -> Q Hymn of Valor -> AA -> W Song of Celerity (Engage) -> AA