First, let me explain my approach for this build:
Soraka is often thought of as a tank, support or an AP carry, but she also can fulfill the role of Crowd Controller and Harasser.
My goal with this build is to strengthen the CC and Harasser aspects of Soraka, which makes a slightly more challenging early and mid games but that it's gratifyingly strong on the end game.

AOE = Area Of Effect
ASAP = As Soon As Possible
CC = Crowd Control - effects that control the behavior of the enemy. Examples: fear, stun, slow, etc
CR = Cooldown Reduction
GP10 = Gold Per 10
HP5 = Health Regen per 5
MP = Magic Penetration
MP5 = Magic Regen per 5
MR = Magic Resistance
MRR = Magic Resistance Reduction - Different from MP since spells from other allies benefit from MRR but not from MP

Pros
+ Starcall becomes a massive CC ability
+ Can easily solo push any lane
+ Great defender late game
+ Spammable abilities

Cons
- Not a good support early on
- Hard to get a solo lane (often denied)
- Very squishy early game and somewhat squishy mid game
- Will be targeted often
- Requires some tactical skills

Consecration
Consecration gives you a 16 MR aura... not that great, considering that any item the enemy has would have 20 MP... but I guess it's better than nothing.

• Starcall is the main point of this build. with it's high spammability (2.5 seconds to start, 1.5 with CR) at LVL 9, you'll be doing 160*6 = 960 AOE damage (+40*6 = 240% AP) plus a 12*6 = 72 MRR in just 9 seconds...
• The MRR works wonders during team-fights, particularly when combined with abilities like Ryze's Spell Flux or Fiddlesticks' Crowstorm;
• The high spammability of this ability makes it a great weapon against Banshee's Veil and other spell shields;

• Astral Blessing is your main support spell and it is the reason you can stay in the middle of the action for Starcall spamming;
• It won't really be of much use until it's at LVL 2, but if you see that you're targeted too often, you can get it instead of Infuse at LVL 8.
• At end-game, healing 485 HP every 12 seconds, it is en excellent support spell.

• Infuse is your Carry Destroyer: Silence + damage every 10 seconds (6 with CR) which is the time it takes you to build a 72 MRR (or 48 with CR)
• It also lets you maintain casters and supporters that are Mana hungry (which is not your case if you follow the item purchase guide)
• At LVL 18 you can easily replenish your allies in between pushes or after a team-fight before trying Dragon or Baron

• Wish is both a great and a useless spell at once:
• You can save a fellow champion that is getting ganked or extend the life of your teammates during a team-fight;
• If during team-fights the enemy acts smart and singles out a hero, wish can heal him for a good amount, but the healing will be wasted on the other team members.

With Soraka, it's difficult to determine a killer combo... if your enemy is a CC, you will have to do an early Infuse, while if he's a melee chaser, you'll want to stack Starcalls for max damage.
Different possible combos:

• If the enemy is a CC, then it would be Q - E - Q * 2, escape then rinse and repeat
• If the enemy is a Tank, then it would be Q * 4 - E - Q * 4 - E, make him chase you
• in a group fight, it would be E (on AP carry) - Q * 2 - W (don't forget to heal allies) - Q - E, rinse and repeat

First, let's have a look at the runes that I selected:

6 * Greater Mark of Cooldown Reduction - 6*0.16 = 0.96 CR
3 * Greater Mark of Scaling Ability Power - 3*1.8 = 5.4 AP
9 * Greater Seal of Gold - 9*0.25 = 2.25 GP10
9 * Greater Glyph of Scaling Cooldown Reduction - 9*0.9 = 8.1 CR
3 * Greater Quintessence of Scaling Cooldown Reduction - 3*2.34 = 7.02 CR

Now, why did I choose these runes? very simple reasons:

• 0.96 + 8.1 + 7.02 = 16.08 CR; when combined with the 4 CR from Intelligence at LVL 2 and the 20 CR from Morello's Evil Tome, we get a total of 40.08 CR. Since the cap is at 40 there is only a 0.08 waste.
  
• 5.4 AP - For lack of better options, you can replace these if you want for Greater Mark of Magic Penetration.
  
• 2.25 GP10; This build is very gold hungry, the more GP10, the better.

1. Meki Pendant to maintain your Starcall.
2. Once you have 605 Gold, transform the Meki Pendant into a Tear of the Goddess and use EVERY opportunity to spam spells: you need to reach the 1000 bonus Mana ASAP.
3. If you are soloing the middle lane, get a couple of Vision Wards (you can use Sight Wards instead if there are no enemy champions with stealth) and use them for early warnings on GANKS and BackStabs.
4. For Soraka, Boots of Swiftness is the way: you are a Chaser and a Harasser, being faster than the enemy is the only answer.
5. If you're harrassed often, go for the Giant's Belt, otherwise stay in the lane and try to go straight for the Frozen Mallet. The reason: 700 Health makes you less squishy, while the Slow property combines with your Boots of Swiftness for Chasing.
6. The Will of the Ancients is quite self-explanatory: 25% Spell Vamp will sustain you against waves of minions, letting you effectively farm/buff. Yes, AOE spells get only 1/3 efficiency, but 3*500 + 3*250 = 2250 Health total in a minion wave, 25% of 2250 = 562 and 1/3 of 562 = 187, so unless you're fighting an enemy hero at the same time, you can kill a whole wave by yourself and still have full Health. The extra AP is also very welcome.
7. The Force of Nature is the last piece on your survivability equipment. The great increase in HP5 makes your hit-and-run tactics all the more dangerous. while it's not enough to sustain you on a head-on battle against a tank, when combined with you slow/silence/heal combo, it will deter any single chaser, with a theoretical total of 297 HP5 (54 HP5 + 0.35% of 2353 + 485 Health per 12 seconds from Astral Blessing)
8. Morello's Evil Tome brings you to 40% CR, the final step on your Harassing equipment. The extra AP and MP5 are a welcome boost to your perma- Starcall.
9. Upgrading your Tear of the Goddess into an Archangel's Staff is an obvious step now, since the extra 3% of Mana into AP is going to hurt.

If you have good teammates or if the opposing team makes the mistake of leaving you alone, you can exchange the Force of Nature for a Rabadon's Deathcap. Doing so will make you quite brittle, so you'll have to avoid direct confrontations... but with a total of 559 AP (340 from items + 30% from Rabbadon's Deathcap + 3% of Mana from Archangel's Staff + 5.4 from Runes) your Starcall/ Infuse combos are going to do massive damage.

When playing a CC/Harasser Soraka you will always have to check your back: your pushing abilities will be great and you're a strong Debilitator and Harasser, so you'll often be targeted.
This build will make you a strong Soraka, but there will still be stronger characters out there, knowing when to fight and when to run is an important skill.
Even though Starcall is a spamming ability, check your surroundings... if delaying it for a second or two will give you last-hits, then DO IT.

The most important thing when playing LoL: DON'T FORGET TO HAVE FUN