Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Starting out, buy an Amplifying Tome or Mana Manipulator depending on how mana needy your partner will be. You will be able to skip buying the Mana Manipulator if you don't need it first. Lane with a high DPS Caster Champion. I prefer Miss Fortune or Ryze, but I've also had success with Veigar and even "speedy melee" like Xin, Warwick, or Yi. This will allow you to stay at a distance and avoid being targeted for the most part. Casting heals as necessary and attempting to KS as many minions as possible. You can use Infuse with a lot of success to harass your opponents. Your lane partner should harass with your assistance to keep opponents cautious about diving for you. Your lane's opponent match-up determines whether you will spend more of your time healing or beating down minions.
With Teleport available, you should return to complete Mejai's as soon as you have the money for it. At that point you and your laning partner can begin to harass harder, with the goal to kill. Call mid up for gank help if you need it. The idea is to get as many stacks of Mejai's as possible as soon as possible. Continue to farm minions for Gold and XP, using Starcall as your "minion finishing move", the timing of which may take some practice.
At level 6, you should begin to pay attention to your teammates for timing your Wish casting. Cast Wish as often as you can with the most benefit to the team, though if you fear an imminent gank, you may want to save it and use it as a quick heal while running back to your nearest turret. Return to base to start building Tear of the Goddess and upgrade to Archangel's Staff ASAP.
By the time you reach level 10, your team will probably start to either push or defend together. You will be key in battle against larger groups of opponents. Do your best to stay near the back of the push, but within range of any high-dps melee for the heals they will need. When I get focussed in team fights there's not much I can do to get out in time because Soraka is slow as hell; I usually pop Wish if it's up in an attempt to get a heal out to my team before dying. Ideally you'll be able to keep your teammates between you and the opposing team, but I haven't had a zero death game at the time of writing this.
From this point out, just keep working on the item build and stick to a high-dps champion or with your team. If you need to, hop to a lane to kill off a minion push but don't stay long as you'll be prime gank material. Don't be afraid to use your Infuse ability against casters when they get in range, and keep your teammates topped off in health if you've got mana to spare.
DISCLAIMER: I haven't looked at any other builds for Soraka at the time of writing this, but I've had great success with this build so far. With a good laning partner and a decent team I have pulled off 2-3-25 and 2-4-20 games without much stress. The real key to this build is being able to farm the money and assists early to get your Mejai's stacks coming in.
Feel free to comment below with any suggestions, I'd love to test out other builds.