My main objective with this build is to create a champion that can support and still take a beating. The biggest problem with support characters is they can be targeted first and killed quickly because they are very squishy. Teams win games by disposing of the support character first. This is where the tank characteristics comes in. When they try to kill you, they will have to waste everything on you. You are practically invincible, considering you have Flash, Shurelya's Battlesong, Crescendo, and Song of Celerity at your disposal to escape bad situations on top of all of your Health, Health Regeneration, Auras, Healing Capabilities, Armor, and Magic Resistance. However, if they choose not to target you, they will have to deal with your abundant auras, heals, and other support skills. You do so much to help your team, they can't afford to let you live. This is a lose-lose situation for the opposing team.

Note: Notice how this build focuses on 3 things:
A. Mana Regeneration.
B. Health Regeneration.
C. Cool Down Reduction.

These main objectives allows Sona to fulfill the role of:
A. Offtank. This is possible due to the Magic Resistance, Armor, Health, Health Regeneration, Cool Down Reduction, and Mana Regeneration provided by all of her items, runes, and masteries. Cool Down Reduction allows her to heal herself more often. Mana Regeneration allows her to keep all her defensive aura provided by Aria of Perseverance up at all times while constantly healing herself.
B. Support. This is possible due mainly to the heals and auras provided by Aria of Perseverance, and somewhat due to Song of Celerity and Shurelya's Battlesong. Also, Crescendo is useful for saving teammates from melee champions. Cool Down Reduction and Mana Regeneration are vital to the fulfillment of this roll. They allow her to constantly heal as quickly as possible and maintain her auras, and allows her to cast Crescendo more often.

How to Utilize this Build Effectively:

Constantly alternate using Hymn of Valor and Aria of Perseverance as quickly as possible without EVER running out of mana.

Benefits of this Build

A. Mana Regeneration is absolutely not an issue with the completed build. It is impossible for you to obtain a low enough amount of mana where you wouldn't be able to use both Hymn of Valor and Aria of Perseverance every 4.2 seconds.
B. Using level 5 Aria of Perseverance every 4.2 seconds adds ((120+.25*20)*1.15 health)/ 4.2 seconds)) = 34.23 health/second to your total health regeneration rate. Your base health regeneration rate with all of your items will be (((4.5+0.55*18+25+30+((265+45*18)*1.05)*0.01)*1.04*1.15 health)/ 5 seconds) = 19.30 health/second. This means your total health regeneration rate will be (19.30+34.23) = 53.53 health/second. For reference, a Cho'gath at level 18 with all his stacks, Force of Nature, Randuin's Omen, Shurelya's Battlesong, a fully completed Warmog's Armor, Eleisa's Miracle, and the proper Masteries and Runes, will have a health regeneration rate of ((7.5+18*.85+45+25+30+25+40+.01*(205+18*40)+.35/100*5*(440+80*18+350+330+920+450+48+26*3+150*6))*1.04 health/ 5 seconds) = 59.02 health/second. Since Aria of Perseverance automatically heals you, you can keep up with the health regeneration rate of a completely decked out tank.
C. Your total health with the completed build is (380+48+18*70+270+330+250) = 2538 health. For reference, the Cho'gath mentioned before would have (440+80*18+350+330+920+450+48+26*3+150*6) = 4956 health. Obviously, your total health doesn't compete with the health of a decked out tank. But here is the kicker. NO ONE TARGETS THE TANK!. They will target you because you are a pain and they will think they can kill you qucikly because you don't have much health. But in truth, killing you will prove to be difficult. They will lose the fight trying to bring you down. So don't be afraid to take some damage, lure them in. Just don't die.
C. Let us look at how much our build costs in comparison to our decked out Cho'gath. Cho'gath's build would cost (1300+2610+3075+2200+3000+1200) = 13385 gold. Our build costs only 9020 gold. This is extremely, extremely important to point out. Our build scales extremely well and is able to be completed quickly. It's benefit/cost ratio is very high throughout the game. We are able to increase our mana regeneration in accordance to our cool down reduction. Instead of trying to obtain AP equipment, we are able to focus our attention on cool down reduction, mana regeneration, and health regeneration to get similar effects. This allows us to amplify our strengths throughout the game, instead of trying to supplement our weaknesses.

Goals:

A. Go to the bottom lane with a teammate that is a good early game harasser.
B. Once minions arrive, constantly attack the enemy champions in your lane with your teammate. Target them with Hymn of Valor and your auto-attack. Hopefully, you can put yourself between the minions and the enemy champions to deprive them of experience, or better yet, kill them a few times.
C. Ignore the creeps, unless they are targeting you, then get them to stop. ONLY LAST HIT MINIONS! With Aria of Perseverance and your 3 Health Potions, it should be easy to sustain both yourself and your lane partner until you have enough money to b and buy a Philosopher's Stone.

Why You Will Be Able to Accomplish the Goals Above:

A. Assuming you buy the items I suggest, you will start with 10 gold when your 15 gold/10 second accumulation rate kicks in at the 1:30 mark. You will need a total of (610+10)=620 gold to buy a Philosopher's Stone. This means, assuming you don't kill a single minion, you will be able to buy Philosopher's Stone by the (1.5 minutes + (610 gold /(15 gold/10 seconds)*(1 minute/60 seconds) = 8:17 mark. This means you only have to be in your lane for a maximum of 6 minutes.
B. If you cast Hymn of Valor as quickly as possible , you would use mana at a rate of (65 mana/7 seconds)= 9.29 mana/second. You regain mana at a rate of (((7.65+3+0.26*9+0.41*9+0.31*9+1.25*3+6)*1.04 mana)/5 seconds) = 6.08 mana/second. This means you will lose mana at a rate of (9.29-6.08) = 3.21 mana/second. Starting with 325.5 mana at level 1, you will lose all your mana in (325.5 mana*(1 second/3.21 mana)*(1 minute/60 seconds)= 1 minute and 41 seconds, or at the 3:21 mark. Now, once your mana is completely depleted, it will take 65 mana*(1 second/6.08 mana)= 10.69 seconds to gain enough mana to cast Hymn of Valor again. This means you can use Hymn of Valor every 10 seconds, even if you are out of mana. Basically, during the 6 minutes you could possibly be in your lane, you should be able to spam the hell out of Hymn of Valor without worrying about mana.
C. If level one Hymn of Valor hit both champions every 10 seconds, which is the minimum rate of usage as determined in Step 1 Section D, you would deal (100 damage/ 10 seconds)= 10 damage/ second. In 6 minutes you would deal (10 damage/ second)*(60 seconds/ 1 minute)*(6 minutes)= 3600 damage. Assuming your enemy opponents each start with 500 health, 3 health potions each, and a health regeneration rate of 7 health/ 5 seconds, to kill both of them in 6 minutes you will need to deal (500 health* 2)+(1200 health)+(14 health/5 seconds)*360 seconds= 3208 damage. As you can see from these sample calculations, you should be able to either kill both enemy champions, zone both enemy champions, or make both enemy champions b back to base before you go back to buy Philosopher's Stone. Keep the pressure on and don't let them do anything. Smother them.

Goals:

A. Immediately use Teleport to return to your lane once you buy your Philosopher's Stone.
B. Continue using the same strategy as before. Constantly target the enemy champions in your lane with Hymn of Valor and your auto-attack in an effort to zone them, kill them, or force them to b.
C. Once you have enough money to purchase Ionian Boots of Lucidity, your Teleport should be off cool down. This allows you to b, buy Ionian Boots of Lucidity, then return to your lane via Teleport.

Why You Will Be Able to Accomplish the Goals Above:

A. With Philosopher's Stone at level 6, you will regenerate mana at a rate of (((7+6*.65+3+0.26*9+0.41*9+0.31*9+1.25*3+6+8)*1.04 mana)/5 seconds) = 8.42 mana/second. Since Hymn of Valor and Aria of Perseverance always cost 65 mana no matter what level they are, you can spam them even more often than before.
B. With Philosopher's Stone at level 6, you will regenerate health at a rate of ((4.5+.55*6+18 health)*1.04)/ 5 seconds) = 5.37 health/second. If you also use level 3 Aria of Perseverance ever 10 seconds, you will have a total health regeneration of (5.37 health/ second)+(75 health/ 10 seconds) = 12.87 health/second.
C. With Philosopher's Stone, your gold accumulation rate will be 2 gold/1 second. This means, assuming you return to your lane with 0 gold and you kill no minions, it will take a maximum of (1050 gold)*(1 second/2 gold)*(1 minute/60 seconds)= 8 minutes and 45 seconds to accumulate enough gold to purchase Ionian Boot of Lucidity. This means your Teleport will probably be off cool down by the time you have 1050 gold.