Hi there, I'm Moderne, your average support main guide writer.
I have two other guides you can find, one for Poppy here and one for Lux here.

Anyway, Warmog's Armor is no stranger to the kits of a few Sorakas in the past, although the effectiveness of such a kit usually requires many corners to be cut to obtain what is usually considered an inferior, and more expensive, Soraka build. This is especially true after Soraka's semi-recent changes to Astral Infusion and Starcall which completely nullifies the cost of healing if you consistently land your Q.

Now landing your Q is easier said than done. It's easy to dodge and its range puts Soraka at risk of being dived by assassins or enemies with dashes who understand to shoot the medic first. It essentially baits you into positioning riskily which skilled players can usually punish.

This build's purpose is to counter Soraka's innate health costs in order for her to cast her heals while also not relying on landing Starcall for sustain. And because healing an ally does not interrupt the healing from Warmogs, you will have an almost constant stream of health which you would otherwise have to obtain from recalling frequently.

Warmogs is fun and, once you hit the late-game, such a build really shines as "pseudo-effective" so take this build with a grain of salt. Ideally, you should know how to play Soraka before reading this guide.

Example gif here.

PROS + Unparalleled Late-game Heals + Deceptively Tanky + Less reliance on Starcall + Enables much safer positioning in a teamfight Lategame, Warmogs Soraka is a monster with nearly infinite sustain and healing due to the combination of Warmog's Armor and Seraph's Embrace. She is also deceptively tanky for a Soraka due to having well over 3000 health.

CONS - Heavy focus on Item Power Spikes - More Expensive than Typical Build - Weaker Midgame - Convoluted Build Path The major issue with this build is the necessity for your items to scale up and the fact that it will take you much longer to reach a point where you are genuinely useful. Usually, you come online well past the twenty-minute mark, usually somewhere in the thirties which, at that point, may be too late.

	Flash: Nearly all champions need Flash as it is the most flexible summoner spell in the game. Considering the fact that Soraka has no other forms of mobility, Flash is essentially a necessity on her.
	Barrier: While often considered a greedy summoner spell, a character such as Soraka needs to stay alive in order to heal her teammates. She is also typically a priority target meaning Barrier is a very good spell to reduce substantial incoming damage.
	Ghost: I love Ghost although it is incredibly unpopular. Whereas something such as Barrier can certainly save you from taking damage, a summoner such as Ghost can also help reduce the damage you take by simply allowing you to kite or run out of the auto range of many champions abilities. It's flexible for running, chasing, positioning during a team fight or reaching an ally to save them. Simply put, the enemy cannot kill you if they cannot reach you whereas they can usually brute-force their way through Barrier, especially late game.
	Exhaust: Essentially your backup pick as a support. Exhaust is exceptional against targets that like to get close or do a lot of burst. The slow it provides is pretty substantial alongside the very high damage reduction. Usually you do not want to be in a positioning where you'd have to use Exhaust, thus making it not as useful as other summoners such as Barrier.

Soraka usually brings Summon Aery with a Resolve secondary purely for Revitalize. However, Warmogs Soraka lends itself much better to a Guardian/ Inspiration rune page.

RESOLVE

Guardian is a keystone triggered by both standing near your allies, or through an ally-targeted ability such as Soraka's Astral Infusion. Versus Summon Aery, guardian has a stronger shield with an added movement speed boost on trigger, but has a noticeably longer cooldown than Summon Aery. The reason we take Guardian over Aery is because 1. The Shield scales off of both Ability Power AND your Bonus Health (which we have a lot of), 2. The Shield is stronger than Aery's (enabling a single smart use of it to block one high-damaging crucial ability vs. 5 instances of scratch damage), and 3. We want Inspiration for Cosmic Insight and we need Resolve for Revitalize meaning we can't fit Sorcery into our rune page without sacrificing our healing power.

The only reasonable choice here is Font of Life which provides your allies some pretty substantial bonus healing through your use of Starcall's slow and Equinox.

Bone Plating is ideal due to its typical strength in laning phase and countering high burst damage. Thanks to how much health you have, you will actually be deceptively tanky through all stages of the game.

Revitalize is essentially the only reason we take Resolve in the first place and why nearly all Soraka builds at high-elos have Resolve somewhere. It's just free Heal-and-Shield Power which makes a massive difference for Soraka.

Cosmic Insight is crucial late-game as having 45% CDR allows you to spam Astral Infusion like crazy. It makes its cooldown at max rank to be approximately 1.1 seconds. Magical Footwear is useful due to being 300 free gold which we need especially because Tear of the Goddess and Rod of Ages needs to be bought early to eventually be useful. Biscuit Delivery is not a terrible choice since it is helpful in lane, provides free mana, and can potentially be sold for some free gold as well, but usually Magical Footwear is better.

Two Adaptive Forces are brought to maximize our AP and improve our heals. Additional Health is REQUIRED in order to reach 3000 Health at Level 13, and potentially earlier.

As a support, you typically have less money than the other players meaning you typically purchase cheaper items that have excellent gold value. The good news is, items such as Rod of Ages and Warmog's Armor have exceptional value to them, and are not THAT expensive. The bad news is they are still roughly 500-600 gold more expensive than your typical support items which, for a support, is a noticeable amount.

Tear of the Goddess

Rod of Ages

Warmog's Armor

Tear of the Goddess is bought early to eventually stack into a Seraph's Embrace. Why is a Seraphs good? Because it gives a large amount of AP, CDR, and, late game, your healing on Warmog's Soraka will not be limited by your health, but will instead be limited by your mana. Now whether or not you wish to build a Tear before Rod of Ages depends on personal preference since you don't actually need the mana early and you can stack it completely fine after completing your Rod. In fact, you could reasonably complete it after your Warmog's Armor and it'd still be fine. RoA, on the other hand, is crucial since it gives a massive amount of health, mana, and AP after 10 minutes which makes building it early critical. Warmog's Armor is the cornerpiece to this build, with the caveat that you need a massive 3000 health in order to trigger the healing passive. Soraka does not reach 1800 health even at level 18 meaning it's better to build it after completing your Relic Shield and obtaining a fully stacked Rod of Ages alongside, possibly, a few other sources. Relevant mathematics are noted in another section below.

When kind of boots should I buy?

Swiftness

VS

Mobility

900g +60 Flat Movement Speed Buffed Recently/More Popular

1000g +25 Flat Movement Speed/+115 Out of Combat Movement Speed Nerfed Recently

Personally, I strongly believe Mobility Boots is better for this Warmogs Soraka build than Boots of Swiftness, even with Magical Footwear crippling your early roaming potential by the simple fact that healing an ally does not put you in combat and, with this build, you really do not want to be hit in the first place.
The additional mobility from Mobility Boots thus allows you to easily navigate within the confines of a fight, retreating back when being chased or in order to recover, before swiftly moving forward and healing up your front line. You are essentially a pacifist-Soraka.

Nerd alert incoming...

I honestly do not know if this math is right because I only spent about an hour lab-ing it out.

In order to achieve Warmog's Heart from Warmog's Armor, Soraka needs a minimum of 3000 Max Health.

From level 1-18, Soraka has 535–1793 Health, meaning, even at Level 18, Soraka would need (3000-1793) 1207 Health from other sources.

Warmog's Armor provides 800 Health. Rod of Ages provides 500 Health after 10 Minutes from purchase. Relic Shield which eventually turns into Bulwark of the Mountain provides 300 Health. In total, this is (800+500+300) 1600 Health.

The Health Shard provides 15 − 90 (based on level) bonus health.

3000 Health can then be obtained at level 13. At level 13, Soraka has 1345 Base Health and the shard provides 68 bonus health making Soraka's health at 1413. This means, with the combined items, Soraka would have (1413+1600) 3013 Health triggering Warmog's Armor.

In total, these three items require a minimum of (400+2600+2850g) 5850 gold, 6700 gold if you purchase Tear of the Goddess during this, and reasonably well over 7000 due to purchasing Health Potions, Control Wards, Boots, etc.

Meaning, ideally, Soraka should achieve ~7000g once she reaches level 13, though she could purchase extra Ruby Crystals with additional gold if she is a lower level. Due to Rod of Ages needing time to scale, buying it as early as possible is necessary, thus you could wait to purchase a Tear of the Goddess until after.

For comparison, purchasing a Spellthief's Edge -> Athene's Unholy Grail -> Ardent Censer -> Redemption would cost around 6900 gold for 4 Fully Build items whereas, with Warmogs, she only achieves 3.5.

Maxed Astral Infusion Base Healing is: 220 (+ 60% AP) (x 1.[Heal-and-Shield Power])

At full build (with Ardent Censer) you will have roughly ~340 AP and 15% Heal-and-Shield Power meaning your healing is: 220 (+~204) = ~424 x 1.15 = ~487 Healing per cast.

You can cast that spell every ~1.1 Seconds meaning your Heals/Second is approximately: ~443 HpS.

Depending on how often you cast (off cooldown vs. waiting a split second), you can roughly cast Astral Infusion about 60-∞ times before eventually running out of sometimes Health but usually Mana.

If you cast it the minimum of 60 times off of cooldown, you would be able to output (60 x 487) ~29220 Health to your allies in a matter of (1.1 x 60) ~66 Seconds before being forced to recall from being OoM.

Hopefully that math checks out or my college will hate me. I also don't know if 60 is the minimum since I feel like it is actually higher.

Will this work in competitive? Probably not. But if a game goes on long enough, you will be a force to be reckoned with. At the very least, it is fun.

tl;dr: don't die

Thanks to This Guide by jhoijhoi for some code. Otherwise this guide would look ugly as hell.

Why does this section exist? Unknown.

Q: In what scenario would this build be useful?
A: It is difficult to argue that this build is better than standard Soraka builds by the simple fact that Soraka's standard build is a bit better in lane and much better in the midgame, while still having incredible burst healing come late game. The only issue is the fact that late-game Soraka runs out of steam really fast if she fails to land her Starcalls (which is risky if the enemy team dives you) and you will frequently need to recall, especially after fights.
That being said, depending on your team comp, Warmogs Soraka could be helpful in team comps that are particularly tanky and like taking extended fights. This compensates for the fact that the Warmogs builds leads to less healing per cast, but more casts overall.
A team that is full of squishies won't need many heals to be topped off, and they will typically be burst down faster than you can sustain them whereas a team with lots of defenses and consistent damage would love a Warmogs Soraka who would otherwise not have enough health to heal them back to full.

Q: Where do I position in a fight?
A: Way in the back, even behind the ADC. Ideally, you should be completely out of the enemy's targeting range, able to quickly walk to the front lines (thanks to Mobility Boots) and heal up a tank before quickly retreating back to safety unscathed.
Due to having so much health, you are actually fairly tanky meaning even a Zed performing a full combo might not kill you in the midst of a teamfight. That being said, if you see any enemies, especially assassins, rushing towards you, you should fall back and let your team peel for you and ideally kill them before returning to the fight. You cannot heal when you are dead.
Usually, Soraka's standard build requires landing consistent Starcalls in order to sustain her way through a fight since it reduces the cost of casting Astral Infusion while also healing herself up. However, Starcall's range isn't particularly amazing, it is somewhat difficult to hit, and good players will try really hard to dive you the second you walk up. Luckily, this build removes the need to constantly spam Starcall meaning you can only do it when necessary, not constantly.

Q: Isn't this build worse because I have less AP thus my heals and my poke are worse?
A: Technically yes. Your AP won't necessarily be worse since items such as Seraph's Embrace give quite a large amount though usually Soraka likes to stack cheap items that also provide utility and Heal and Shield Power so, overall, your burst healing (heal per cast) would technically be worse than usual. You overcome this, however, with your sustained heals somewhat like how an adc does sustained damage.
And sure Soraka's poke is technically worse but you usually start the game the same as a typical Soraka (minus some rune differences) meaning, during the laning phase, your Poke should see no noticeable change. And if you are picking Soraka because you believe her "mid-game poke is awesome" then you are choosing the wrong champion.

Q: Why do you type so much and are so wordy? I hate it.
A: Yea. My teachers yell at me all the time for being too wordy. Too bad.