My summoner name is Unreal Sona, and I'm playing on the EUNE's server.
I'm actually a challenger, and I only play Sona in my rankeds.
I earned world rank 9 with Sona in lolskill.net with this build. :D

Well, this build is works well in the late game, because you need to get your essentials items: The Seraph's Embrace and the Ardent Censer. The Seraph's Embrace helps you to lower your skills cost, and it provides a significantly amount of mana, that allow you to use your skills more efficiently. The Ardent Censer's is especially made for Sona, because if your team is good at the teamwork (they can stay close to others), you can boost your entire team with this item abilities (attack speed + on hit damage boost) + your abilites buffs.

The laning phase is not too much hard. You should try to poke the enemy, if can do it safely. To do this, you need to look on your team minions, which is low on HP, because the enemy ADC probably wants to attack that, especially if it's a cannon minion. It's a good time when you want to poke the ADC, because they targeting the minion that time, take your chances, and try to hit him with your Q and basic attack.

Stay behind the minions!
If there's not too much minion in front of you, and you playing against a hooker champion like: Pyke, Thresh, Blitzcrank or Nautilus you have to play safe. Always watch out, how much minions you have around you.

Always try to get some gold!
If the enemy is a much stronger than you, then you should use your Q from a far range to get some money if it possible. Your Q range is high, you can use it any time to get some gold by poking.