Well, first of all, this build is not about winning, its about making the other team scream in hate and making them targeting you first, last and always, and you not been there when they try to take you out.

Runes:
Not much to say here, Its about CDR and Movement, one could go for more in DEF-tree if one would like to, but in my experience, Magicresistance is the only thing needed.

Items:
First of all, This is not a cheap build, but if you play good, you will have assists from all over the map from lvl 6+ and therefore can afford it all.

I take a tomb at the start, just to get some AP going instantly, makes life abit more fun for everyone.
Then I make sure I get mejas before lvl 6, so I can start "stealing" assists worldwide for it with my Ulti.

After that I go for the boots, all the way...the impressive 1K it costs.

After that one can star to take it easy, farming to around 1-3K and then go buy next item.
I placed the things in the order I get them, the way to the final Item depends on the enemy team.
I go for the Giants Belt, Directly after the Boots has been made, thereafter I pick what I can afford when I happend to be at the base, aiming to get the Rylai done asap so I can slow the enemies for my friends, and to get a "get out of jail" free card =)

Why take Silence instead of heal at start?
Simple, when the enemy can't cast that spell they wanted and takes damage, they run the other way, only in real self-confident players one have to have heal....and that´s why one has heal as summoner spell.

Sometimes I take SC abit earlier, instead of one of the heals if I feel the need to help my partner to take out creeps.

At lvl 1, I prefer to go to top.

Getting the Silence/MR.
And the Tomb.

Then I just support my partner in the farming that starts, and the most important part, Harass the enemies, silence them, take a shot at them, all the time, if they go for you just heal, silence and attack, they will fall, if they got a stun, make sure your partner gets into it instead of you by letting him/her engage, and be prepared to heal.

At lvl 5 I make sure I get my Mejas, since I know that at lvl 6 the farming for stacks can begin with the Ulti.
And here the fun begins.

I stay in my lane until the towers are gone, then I and the partner starts to gank.

I care not for neutral buffs at all, but if I would, the redbuff would be prio.

When getting into teamfights, I run in, make a SC to make sure the enemies are slowed and I will recive a stack for Mejas when they die.
After the SC, I run out again, and starts to heal the ones in need, and silencing the most offencive heros of the enemy team, picking off the weakest of them with a SC and a silence if I have the possibility.
When the fights gets to an end, I use the Mastery Heal to get my friends up to a base of not been creeped =)

If you play this right, the enemies will write things like "**** you Soraka" "Lucky bastard" "HOW THE ****?!?!?!" all the time to you...and thats why Im doing it, play offencive healer and at the end, with full stacks in Mejas you have 800+ AP and heals like a god, they will Scream and scream in hatered at you, thats when you know you have won and played this character right.

Best wishes.