This guide is still unfinished, I simply wanted to see how people would react to it and for tips on how to improve it seeing as this is my first guide. I am also aware that Some people do not agree with ap carry Sona.

I am aware that most people will want to play each champion slightly differently and when a person makes a custom build it gives them a sense of ownership making play much more enjoyable but getting to the point of making your own builds normally comes after you have played a champion a few times. Getting the experience may be unenjoyably though due to teams that are unwilling to communicate or have poor map awareness.
So this is a multi-playstyle guide for Sona that I hope many of you active, veteran and new players can enjoy.

Sona is a very squishy champion but despite this she has great lane sustain and amazing burst damage often allowing her to outdo some of the better midlanes such as Brand. For Sona to be a good midlane or even a possible solotop you will need higher base stats and for this we will need the listed runes with the following reasons:
Greater Mark of Magic Penetrationx9
Greater Glyph of Scaling Magic Resistx9
Greater Seal of Armorx9
Greater Quintessence of Ability Powerx3

As stated Sona is quite weak to start but due to the seals she is able to take more early physical hits from champions and minions making her early sustain that much easier. The glyphs are magic resist per level making playing against mages in mid not so unbearable due to the nukes they can throw.
Sona's passive is Power Chord which does a burst of damage after three spells cast (not including her ultimate) with varying effects. To make full use of Power Chord in terms of damage it must be used with Hymn of Valor this is then increased though the magic pen marks.
The Quintessences are flat ability power giving sona an extra burst of damage and healing.

- A small boost to ability power, may not seem like much but early game every point into your stats makes a difference.

- A small cooldown reduction of 4%, as stated already these stats may not seem like much but they add up when look at as a whole.

- A great point well spent, 10% magic pen will increase all spell damage you do by ignoring 10% of the target total magic resistance, this will effect your ultimate, your Q and your passive's damage.

- Using a mix of both magical and physical damage despite being ability power based this is a great choice, it will increase all damage you pump out by 1.5% allowing for that extra bit of damage into every burst.

- With a slow climb of 1 ability power per level to a cap of 18 extra this is just a small boost to your maximum potential.

- This is the point you wanted the most from this build, ot provides 5% bonus to your maximum ability power, increasing all boosts already gained and will gain to allow you to reach a high amount of power. This will help out in both healing and damage.

- This gives you an extra 6% damage to targets below 40% health, if you can take an enemy off guard and get them to this point killing them off will be no problem.

- Increases mana gained by 20% from Clarity and gives Clairvoyance an extra 2 seconds in duration.

- A 7% decreasment to your total deathtime, as a support you are bount to be targeted sooner or later, why not shorten the time?

- With an extra 12 mana per level with will both boost your ability to cast but also stack into adding up into your Archangel's Staff with a bonus of 216 mana.

- Provides you with a bonus 3 mana regeneration, this grants you with the ability for early casting potential by granting you mana over time.

Core Items:
- Archangel's Staff allows for Sona's to keep casting due to the growing amount of mana and a mass amount of ability power gained through total mana.
- Morello's Evil Tome has all the stats a support will need from ability power, mana regend and cooldowns.

Suggested build order:
X3 - These items to start off will give you the early mobility one needs to move in and out of combat and or range of an ally, it also makes it so you do not need to waste mana on healing yourself outside of combat.

- Due to Sona's Natuarlly high burst damage one will not need to focus eary on into a high ability power item but more into an item used thoughout the game, getting Tear of the Goddess will allow Sona to build up the mana stacks needed for later and will allow her to sustain lane through more constant healing and attacking.

- Given that Sona's abilities are not known for their short cooldowns but neither are they to long, but despite this cooldown reduction is needed along with the movement to escape unessassary damage.

- By this point Sona will need higher damage and healing and to gain this Rabadon's Deathcap is the best choice item since it will give the largest amount of easily gained ability power, it is also better to go for this item considering that in this point in time the bonus ap will be much greater than that gained from upgrading your Tear of the Goddess.

Kage's Lucky Pick - Even if your income is great this is the next item you should aquire, you had probably just spend all your gold on Rabadons and will need to gain money fast to finish your build. This item will also be upgraded later.

- by now your Tear of the Goddess will have a large amount of bonus mana which is needed for a mass ability gain, for the time this is the next best choice.

- This item has great value for Sona since you are already comboing your physical attacks with your spells to abuse your passive's damage this will add the extra boost needed to ensure that your target dies.

- You no longer have a need of the money gain from Kage's Lucky Pick so it is time to upgrade it, this will provide cooldowns, mana regen and abilitypower.

The last item is left open for your own personal choice, depending on what you think will fit best depending on how the game is going.

Other possible items:

As a sololane you will want to start off with Hymn of Valor for the damage output but as a duo you will want Aria of Perseverance for the survivability.
If you find you are dealing more damage than your opponent level up Hymn of Valor if not up your heal.
At level four is when you will want Song of Celerity, I find this is the time most people will try to gank you first, this ability will give you the speed to escape and will also help you stack your passive.
Now you should be getting your heal and attack in the order you see fit and your ultimate whenever possible due to the high damage it deals in addition to the stun, you can use this spell for ruining the chances the enemy has in almost every teamfight along with kill most people 1v1 because of the damage you can deal out before the stun ends.

Due to still being a support the best summoner spells should be ones to fit that roles, the ones I take are as following:
Clairvoyance; this is used for checking before you ever extend for a kill, checking jungle locations, checking both dragon and baron, showing the safe path to run through from enemies and so on. If your team is good they will know how to abuse this.
Clarity; This is more for early game, it allows you to refill your mana without the need to back, this buys you time before you back for your Tear of the Goddess. This spell is also used to fill the mana of your allies or for when your mana bonus from Tear of the goddess is still low.

Other good choices:
Heal; When you find that you and your allies are taking to much damage this is a great thing to have, it can be activated to give your team just enough health to win a fight or enough to be healed back up by you.
Promote; People often claim this useless but it enables a lane to push itself forcing at least one enemy to go deal with it.

Counter to AD carrys eating your team mates would be to use your passive with the ability Aria of Perseverance to both heal your ally and reduce damage they take, in addition to this the enemy deals less damage. If this does not work you can simply use your ultimate.

If you see your jungle coming in for a gank charge up your passive to two stacks then move into position and use your ultimate after your jungle enters lane, this gives them time to get in and deal some damage, if the enemy does not die activate your speed boost and slow them with your passive to ensure the kill for at least one of you.

Unexpected or unwanted teamfights can be turned around by holding back healing up your team and moving in to fire off attacks, then when the time arises activate your ult getting as many enemies into it as you can.