Hello, this build tries to explain how to play Soraka the way I think it's the best, supporting your team, specially your AD carry. I will add a Key idea (in italic) in the end of its section which will be the essential stuff, try to follow them !

As a guideline, Soraka will change his gameplay in these 3 phases:

• Early game: During this phase Soraka has to help his AD carry to farm as much as possible. She should never last hit (only last hit -NEVER advance your lane- when your ad carry goes to buy and if you don't last hit that minion none will)
• Mid game: On this phase Soraka has to pay TOTAL attention to everything around her. Since her W spell has a lot of CD she should heal to the one who really needs it and never waste it, the same applies to his ultimate, his ultimate can totally change the outcome of a fight so dont waste it, you have also to pay constant attention to the map since you can save someone from dying and even win some assists if you use your ultimate wisely.
• Late game: Ok, Team Fights time. By now you should be able to spam your Q on the fights, try to silence the enemy when he tries to scape (you can prevent them from flashing for example), and also keep your team's HP up, which is your main function. If you already got your Deathfire Grasp use it wisely (for example when your team is going to catch a guy with a big percentage of his HP up).

So we can sum up Soraka in 4 key ideas:

You have to make your AD carry farm and try to avoid theirs from farming.

Yes, you are the main warder, even though your team should help on that, it's your task to ward the map.

Absolute attention everwhere and paying total attention to your CDs since they are long.

NEVER take a sure kill from other one. Everyone else in your team needs the money more than you do, so unless the enemy is going to scape, DON'T take the kill.

As you can see I go for flat stuff, this is because that flat AP and flat MP5 helps a lot during the Early Phase, your items will supply your needs when the game advances so perhaps you can save/kill someone thanks to this runes. I know the yellow ones with mp5 may seem a bit unnecesary, and if you feel so, change them for AP or any other things, I like to use those since I'm pretty aggressive using Soraka so I spam a lot my E on both my AD carry (so he can harrash/last hit better) and the enemies.

The key of my runes is :
Flat runes will work better since my items will do the rest when the game goes on...

These are a pretty basic support runes, of course you need to improve your CV and your flash since both are important.

9 points on attack just to get the magic penetration since our Soraka will deal considerable damage during the game, specially in late game when she will Q spam.

This section may be controversial since every game is a different world, but there are some guidelines to consider besides the sequence I already gave:

• We start with 3 wards, health pots (no mana pot) and the mana reg stuff. We will evolve the mana reg stuff to philosopher ASAP, boots shouldn't be needed before.
• Soraka DOESN'T FARM but we need cash so we rush Hearth of gold just after the philosopher, if you don't feel comfortable without boots you can buy them (the basic ones) before the Hearth of gold, this will depend on the game itself.
• It's important that you buy TONS of wards once you have your 2 Gold per Ten items and the basic boots, an Oracle will be needed as well, but you have to know when to buy it. If it's obvious they have several wards you may even pay your oracle killing their wards !
• On the background you should be building your Deathfire Grasp, which will convert you into a pretty decent DPS burst, but there should NEVER be a lack of wards because you are building other items.
• While the game advance you should improve your Philosopher Stone to Sureya's which's use should be use wisely (1 min CD so you can even use it twice on a TF, specially to catch a scaping enemy when you are winning.
• If the other team is dealing lots of magic damage you should have bought a Negatron which you'll turn into a Banshee's Veil. Otherwise, if they are dealing lots of physical damage buy a Chain Vest. You have to choose when you buy what, but perhaps if you get your Negatron or a Chain Vest @ mid game, it will prevent you or someelse from dying !

These were the guidelines for the items we should buy. Anyway I'll give you 2 golden rules:
Your items are less important than warding the map or taking their wards out.
You need those 2 gold per time items ASAP, they are your main gold source.

This section will may controversial as well. As you may see I'm not leveling my Q until the game is pretty advanced... Why is that? Easy, that spell advances your lane, and you generally don't want to do that. But if the other carry/support are close to dead all the time and you feel you could kill them with a few Qs, perhaps you should give a point to it @ lvl 4-5.

And yes, only with your E, without your Q you can be a BIG pain in the *** for their farming lane, if your carry helps you as well.

It's also a good habit when the game is starting to ask your AD carry which skill she wants you to get first W or E, take the one that makes him more comfortable.

As you can see my Soraka is planned to be dedicated to his AD carry during the early phase and then she's just meant to be a pain in the *** for the other team.

You can make a more aggressive Soraka in the beggining, but I think this could affect your carry's farm. In end-game with those items (specially the Grasp) you should be pretty annoying, but remember your role is:

To keep your team alive while disturbing theirs as much as possible.

NOTE: This is the first guide I upload, I will keep changing it (replacing the names I wrote with links to the items/skills, etc. Anyway my ideas are already shown, that's why it's already uploaded.