The meta for League has been stable for a long time. Top lane is for those who do best with both gold and EXP, usually requiring an escape for their long lane. Junglers are given the farm that spawns in the neutral jungle and are expected to provide pressure(usually in the form of CC) to the lanes. Mid is for those who require both levels and gold but lack the escape so they're given the shorter lane. Bot lane is filled with an ADC and a Support, this is done because ADC's usually scale much better with items than they do with levels, but they lack and escape with their long lane. That's where the support comes in. You're the odd one out, your job is to do as much as possible with as little as possible. That's what this build is designed to do. With almost no farm at level 12 you can do upwards of 1'300 Damage. Both Manamune and Archeangels are over 200% gold efficient when they're fully stacked with each other. As Sona you can even make use of every stat on them. Relevant and Easy.

Sona has excellent range on her Q which gives her a rather interesting method of farming. You can farm champions. A simple rotation of AA>Q>PC will keep you fed and keep the enemy backed off while your ADC farms safely.

Now down to the mechanics. Sona's Q acts as an auto attack reset which gives you great burst for the first few levels. you should charge up your passive to 2 stacks while in base. When you first get in range of your target your skill rotation should be Auto Attack => Q => Empowered Auto Attack. That rotation will put almost anyone(besides Leona) into the danger zone allowing Bot lane to free farm until level 3-4 when they healed it up again. While they are backed off use your W heal to charge you passive and keep your ADC topped off, and remember don't forget to ward

As any support should do after you build your first tear, and upgrade your spellthieves once you should rush a SightStone. Vision wins games. If you want a comprehensive guide to warding I'd suggest this guide on Leaguecraft.
http://derpy.me/wards . Now The slight issue with going both Archeangels and Manamune is that they wont stack charges, one of them upgrading will sap all the charges. So after you build your sightstone try to finish your archeangels stacks as fast as you can because then you can start stacking your next tear. While you still have your archeangels and its taking a while to stack you should build RoA as it provides some much needed tankyness and gives you something to do while you wait for the next item you need. Don't bother upgrading the sightstone until you're completely full build and already chugged and elixer it's really not worth it.