Soraka has all the tools you want in a support. Healing , mana regen , aura passive ... global heal. But to take advantage of these abilities you need to build ap ... making her very vulnerable and keeping her away from most of the support items (aegis , philosopher stone).
So this build focuses on still giving her items that support the party while allowing her to have some ap for the summoner to chose what to do with. Youll notice abysmal and frozen heart both have passive auar's , the mage's staff has an active spell to get a shield ... and the iceborn gauntlets do area effect slow. So with this build and good positioning you should feel soraka makes a much greater effect in team fights than just a normal tanky support build and the defense of this build should allow you to sustain one volley of attacks which you can recover because your w spell will heal alot of health.

Start with a ferry charm some sight wards and red pots. You will want to b when ever you get enough money for tear and it is a good time (low health, low mana). Once you have a tear you will hardly have mana problems. Your lane should sustain pretty well with soraka. If you are getting mauled then get q fast and help clear under towers. This can be done so that you dont take cs. If you are mauling then build e. In a standard even poke lane just go w for better sustain.

With soraka the bot lane should be looking to farm not get kills. With the teleport soraka can get back to lane even if you make a mistake or get ganked early. The ignite should allow you to get a lucky unexpected kill or hold the tower early game. Soraka does good early against mana/spell based adc's (varus, vayne). Soraka does poorly against hook supports (thresh, blitz crank, leona). But for the most part you should be worried about warding the river and just sustaining early game. If you get pushed back early get 1 level on q and help your adc with clearing minions so you can help dictate where the lane stalls(these are advanced raka tatics , using your q spell to help with cs not steal cs).

Now with the second tier of items and a full stack of spells and the teleport up (back up) it is time to show your knowledge of this charcater and the game. A skilled player will use the whole tool box to position raka in the right place to save a tower, dive a tower, get a kill , slience a nunu ult , keep a player alive at top while you sustain the mid lane's mana ... etc. There is alot soraka can do but the key is to keep her alive so position yourself correctly and watch your mini map ... you will notice that sight stone is not in the build so please keep 2 wards on the map at least.

Giving soraka ap will help her sustain her team and make her more of a threat in battle. If you are losing you might want to take frozen heart at this time ... but if you are winning you might want to go rabdons now. Either way ap helps raka be more raka. One personal note is that the 20 cdr from fronzen heart is really helpful ... no matter how much ap raka has she is not a burst character but with good cdr you will win 1v1s that most would not think becuase you can get 1 more w in and one more e in .... then you use ignite ... LEARN THIS GUYS ONE SECRET TO AP SORAKA ... AP MIDS HATE HIM!!!! .... its a party, its a party , its a party