I'm still working on the guide over time, but this a comprehensive view of what my Soraka looks likes for ranked play.

Okay let me explain my runes:

For Marks I go with Magic Penetration so my Starcalls and Infuses do just a little extra damage, and also do true damage to creeps/minions.

For Seals, I chose specifically for my Soraka runepage, Flat Mana Regen. I need the extra mana regen at level one to keep the Starcall spam up so we can ensure a kill, if Soraka draws first blood, it usually crushes the enemy teams morale.

For Glyphs, I go with Cooldown Reduction Per Level, With Death Fire Grasp I have just under max cooldown reduction at level 18.

For Quintessences, I choose Flat Ability Power, Because the 15 extra Ability Power at level one gives me 30 AP with Doran's Ring, so I don't need an AP item so quick, and it allows my level one heal to be not quite so ****ing terrible.

I run with 9/0/21 Masteries.

Offense:
1/3 Deadliness, Since I don't use exhaust or smite, this is the best use of the mastery point, Soraka does have a 2% base crit chance so whatever.
3/3 Archmage's Savvy, Pretty self explanatory, AP per level, whats not to love.
4/4 Sorcery, CDR, Gotta spam dem Starcalls, heals, infuses...
1/1 Archaic Knowledge, you don't get a whole lot of magic penetration as Soraka, so this helps out with anyone who has MR before you've Starcalled them 8 times.

Utility