The build is just mandatory for publishing a guide. Ignore it if it bothers your.

This is a quick reference meant to help the actual game play of any Sona player. There are a bunch of things that aren't obvious just by reading the tool tips and my hopes is that by posting what I've found I can help people play a little better.

There are tons of viable builds and guides and people who insist their way is the best but this guide isn't about that and I don't care to get into internet debates of that type at the moment.

1. Fountain stack:
Use the fountain to get free stacks of your passive before you head out. I recommend 2 stacks so you can still use your auto attack without wasting your Power Chord.

2. Tower Harass:
Sona can harass enemies under their tower with her Hymn of Valor by timing her attacks between tower shots and backing out of range before the towers fires its next shot.

3. Spell range:
Mouse over your spells to see their radius. Knowing the range of your spells on this champ is extremely important. (The range on Song of Celerity is bigger than indicated).

4. Power Chord = Spell:
Power Chord counts as a spell and will proc on cast items like Sheen, or Rylai's Crystal Scepter.

5. Walls and Spells:
Your Hymn of Valor Aria of Perseveranceand Song of Celerityspells will still auto target champs on the other side of walls as long as you can see them and they are in range. This allows you to flash to safety and still contribute to a fight.

6. Minion sprint:
Your Song of Celerity seems to effect minions as well as champs.

1. Move+Cast:
Hymn of Valor Aria of Perseveranceand Song of Celerity are all castable while moving and do not cause you to stop unlike other spells or auto attack.

2. Global Cool Down:
Sona has a 2 sec global CD after she casts her Hymn of Valor Aria of Perseveranceand Song of Celerityspells. This is reduced by CDR.

3. Aura Persistence.
The aura of your spells will stay for 2 sec after you switch.

4. Hymn of Valor targeting priority:
Hymn of Valor will hit the closest champs or the closest minions if there are no champs in range.
Or if it's easier to remember:
Closest champs > Farther champs > Closest minions > farther minions

5. Aria of Perseverance Targeting priority:
Aria of Perseverance will heal Sona and the teammate with the lowest % of their total HP remaining that is in range.

6. Don't forget
your Crescendo counts as a spell toward your Power Chord spell counter.

7. Auto attack auto fire:
When Sona has her Power Chord ready she will not automatically choose target and attack. When she does not have Power Chord up she will automatically choose a target within range and begin to attack. This is important when hiding in brush since attacking will make you visible to the other team.

Your Crescendo has a travel time.
The travel time combined with inherent latency makes it so that if an enemy is at the end of your [spell=crescendo] range, and running away from you, the spell will not hit if they leave the area of effect before the spell reaches them.

Your Hymn of Valor and Aria of Perseverance also have travel times, but they can not miss. Once you have cast the spell it will automatically follow and home in on its target no matter how far away they run.

CDR:
Sona revolves around CDR. The past few patches have reduced spell cost and AP ratio to the point where CDR is by far the best stat. If you look at the gold cost to increase your spells effects by 40% though AP vs CDR+mp5, there is no contest, even before you factor in the effects of more 2 sec persisting auras, more power chords, more ults, and sprints.

Survivability:
Sona is fragile. Without some survivability items she is incredibly easy to kill even when you are avoiding contact. You need enough survivability to live though an enemy dash/leap/flash attack/CC. If the enemy knows you are a free kill they will take advantage of this every fight.

Flash:
OK this is just my opinion but I feel Flash is by for the most useful choice on Sona. It can be used to escape giving distance and allowing you to run with Song of Celerity.
Being able to escape consistently ends up deterring the enemy from attempting to dive in to kill you.
It can also allow you to stay in the back of your group in a team fight and flash in to land your Crescendo when it is most needed and unexpected by the other team.

Hope this was useful

If I made any mistakes or left something out let me know so I can correct it.

If people actually like this and would like for me to put up a full guide with builds items and all that good in-depth stuff let me know and I'll try to find time.