PLEASE READ

I am hugely thankful for the constructive criticism I have already received and after reading the comments I felt I needed to add this.
Firstly I am dyslexic so there will be grammar/spelling issues, if you spot any, just point them out and I'll sort them, thanks. :)
Secondly, it is a work in progress guide, as it has already been pointed out there is much BBcoding that needs to be done. It will be done but might take time as coding is really not my thing!
And last, some suggestions have been made in terms of alternative build items that I think are fantastic! I'll gladly include your ideas if I think they work with my build.
Thanks for reading.

Before you go to all the trouble of reading this build you want to know it works, right?
Helarolen

Considering this build has only been on here for a few days I'll put a couple of my scores up until I get some more people letting me know how they did :)
Me

Firstly, I do not claim do be the best Janna player ever, I do not claim that this build is the best build ever. In fact I'm pretty bad at League, or I was until I picked up Janna. I tried using other builds on this site and I felt like I was only useful when I was standing there throwing out my auras, I might as well not even have abilities, I'd try to shield people to save them from a Karthus ulti and they'd still die. I'd heal the team in a team fight with Monsoon and they'd still all die. It wasn't because I was building her wrong, I was using highly rated builds. So I decided to look at alternatives.
Building full AP results in meaningful heals, painful Howling Gales and Zephyr, and best of all, epic life saving shields. Now instead of simply lingering near your allies to give them auras you can be a proactive force for helpfulness.

Before you play a champ it's important to know what they're good at and what they're not good at, for example, it's no good playing le blanc if you feel like tanking. So I've laid it out clear and simple, what Janna is good for and what she's not.

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Pros + Excellent shields! + Has many tools to help allies escape nasty situations + An ulti with huge amounts of utility

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Cons - Squishy - Will be focused frequently - If something goes wrong you will get the blame for it, whether it's your fault or not!

Janna has a combination of interesting, and useful abilities that make her really fun to play.
Her abilities can all be used to assist in a kill or help your ally escape, this along with her manageable cooldowns is why she is my favorite support. She would be my favorite champ if cottontail Teemo wasn't so cute! Im going to talk about her abilities and the different ways to use them all.

Tailwind

3% bonus move speed for the whole team which is lost while Janna is dead.

Howling Gale

Howling Gale is a skill shot which you place on the ground, you can either release it at once by pressing Q again or you can let it charge so it travels further and deals more damage. Howling Gale will eventually release by its self if the summoner doesn't press Q.
The way I see this ability being used most commonly is to knock up an enemy in order for the team to get a kill. You just need to be careful since with this build you'll be dealing some damage with it. That's a great use for it. But there's so much more.
When starting a game it can be used to check the bushes on top and bot lane. Not many people will stay in the bush after you've sent a few Howling Gales in there, particularly not if you're using AP runes as those early Howling Gales hurt!
It can be used to counter a gank. If you're attacking the enemy's outer turret and you get ganked from the jungle a well placed Howling Gale quickly followed by a Zephyr will hopefully give your ally time to escape.
If your team engaged in a team fight and it didn't go well you may decide to all run back down mid. A good Janna will have the sense to leave a Howling Gale charging up to slow the enemy as they chase you. Of course it won't hit all of them unless you're really lucky, but there will nearly always be someone not paying attention who will walk straight into it.

Zephyr

This is a single target ability which slows and deals damage. Its passive gives you and a move speed boost and allows you to ignore unit collision. When being chased this can prove very useful as you will be simply be able to glide over your minions and the enemy will be slowed by going round them.
This ability is pretty straight forward. It slows people and can be used in an offensive or defensive manner. You could slow an irritatingly speedy Teemo so your team can squish him, or you can slow an enemy to give your teammate time to escape. It doesn't deal very much damage at all early on, hence I only put one point into it, once you have some nice AP you'll be surprised how much damage it actually does.

Eye of the Storm

A single target shield which also gives an attack damage boost.
This is why I love Janna. This is why building nothing but auras is such a waste. This shield is fantastic. I'm sure someone is going to point out to me that another champ gives a far more powerful/long lasting shield, I don't care. It has a manageable cooldown, low mana cost and incredible effectiveness, whats not to like?
A cheeky tip from when I was a Janna noob: If someones about to get a kill and deosnt need any help at all, shield them anyway. It's an assist for you ;) It's like 'assist stealing' except people don't get angry about it like they do with kill stealing, more gold for you, a kill for them, everybody wins!

Monsoon

This ability heals all allies within a radius of Janna and knocks enemies back out of the area.
It is fantastic.
It can be used to turn the tide of a team fight. If it looks like your team is going to lose, pull that ulti. You can get the enemy to back off for a second and give your allies a heal. Be careful because it will get you targeted. Although I rarely shield myself in a team fight situation, in this instance I do because if I get focused I can continue to heal for longer if I have a shield.
Sometimes during the final push you may have killed most of the enemy team but your allies might not have quite enough health to finish the rest of them off. It is perfectly acceptable to use your ulti simply as a heal without making use of its knock back properties.
It's an excellent counter to a gank. As I mentioned previously a well placed Howling Gale followed by a Zephyr should save your allies, however when being ganked by large numbers of enemies a Monsoon might be your best bet.
A more advanced use of Monsoon is to use it to push a low health enemy towards your team, obviously no one is going to thank you for pushing a full health Dr. Mundo into the path of a low heath Anivia but if used with caution and consideration (and perhaps a Flash) an enemy can be isolated from their team and blown into the center of your team to be quickly disposed of.
Be Careful!
At first you will find yourself accidently blowing enemies away when your team could have killed them, that does not make you popular, believe me!
You may misjudge your position and use monsoon to assist your team in an escape without realising you're actually blowing an enemy towards your team at great speed!
Getting these things right takes time, practice and confidence. A lack of confidence means you'll be making your choices a couple of seconds too late and what might have been right a second ago could be completely wrong the next. So if you think pulling a Monsoon might be a good idea, stop thinking and do it, because you're probably right!

I'm going to take you through each item, the reasons for buying it and the alternates. I've also broken it up into what I generally buy each time I recall. If you're not using the same masteries as me you might not be able to buy the same starting items. I haven't included wards in the build because I tend to just buy them whenever I have spare gold

Amplifying Tome, Health pot, Mana pot.

The Amplifying Tome gives a nice AP boost and builds into your kages lucky pick which you need to get early so you can be collecting gold because I know this is an expensive build, and its good not to be taking your laning partners gold.
Health Potion because you don't have a heal yet (and personally I never use my heal to heal just myself)
Mana Potion because again, I personally never use my Clarity to just give mana to myself, I always make sure my laning partner needs it too in case a situation arises where they need mana and don't have it, I'm more likely to have a Clarity saved for them.

Kages Lucky Pick, Boots of Speed.

kages lucky pick
kages lucky pick is needed so you can stop taking your partners farm, let them farm up and get their items. It also gives some AP
Boots because everyone likes boots :)

Philosophers Stone, Ionian Boots of Lucidity

Philosopher's stone
philosopher's stone gives you regeneration which is really useful to Janna and some more gold which is always nice!
I chose Ionian Boots of Lucidity because it means I can use my shield more often, and with my philosopher's stone I have the mana regen to keep using it. Sorcerer's Shoes are an alternative for a more aggressive Janna build.

If things are going well build a Rabadon's Deathcap, if not go for Athene's Unholy Grail, I'll assume it's all going well and talk you through the Rabadon's Deathcap first.

Needlessly Large Rod

A nice dollop of AP whats not to like?

Rabadons Deathcap

An even bigger dollop of AP!

Fiendish Codex

It's got some nice mana regen and some useful CDR so you can keep throwing out those shields.

Chalice of Harmony

Got some useful stats and you might want to buy it before the Fiendish Codex if the enemy is heavy AP.

Athene's Unholy Grail

I love this item. As I mentioned previously, if things aren't going well, eg you're running out of mana then grab this before Rabadon's Deathcap. This item has a bit of everything and playing Janna has become a bit easier since it came out.

Usually you will have supported your team to a glorious victory by now but if not there's a lot to play with.

Blasting wand

A bit more AP, something to spend your money on while saving for bigger better items.

Rod of Ages

Finally time to look out for yourself! If you find you're dying a lot you might want to get this sooner because feeding is the last thing you want to be doing!!

Rylais Crystal Scepter

rylais
I love the fact you can hinder the enemy's escape by knocking them up then they come back down to earth just to find they're slowed! There is no escape! If you really insist on having auras a Will of the Ancients or an Abyssal Mask] would be a smart choice if your team is heavy AP. Alternatively a Frozen Heart when facing an AD team will help you by making you less squishy, giving you more (much needed) mana and getting those cooldowns lower so you can keep the shields coming!

Morellos Evil Tome

Build that kages lucky pick into a Morello's Evil Tome. I would advise making sure you have a lot of gold saved up before you buy it since you'll be losing one of your gold per 10 items. It gives you some nice mana regen and some CDR which if you went with Sorcerer's Shoes you might want.

Mastery Tree

Runes

For an AP build it's a pretty obvious choice of runes, I suspect cooldown runes would be very valuable but unfortunately I don't currently have the IP to experiment with that. However AP runes are very useful early game when you haven't bought any AP items, it means your can still give useful shields without AP items. Again, I'm not saying this is the best/only way of doing it but it works for me.

Summoner Spells

Generally I chose to take Clarity and Clairvoyance. However a Flash is always a useful spell for a support as it enables you to get somewhere quickly and assist your team, a Ghost is good for the same reason. I take Clarity because I like to use Janna's abilities a lot, because I think they're great! A better player may not find that they need Clarity as there is good mana regen in this build and a better player would probably know when a shield is not needed, unlike me, I just like to be helpful and give everything a shield :)
Clairvoyance I find very useful. When playing in a premade team I simply ask my friends just to ping where they want a CV, when in solo games I make sure people are aware that I have CV and just need to ask and I'll use it. Besides its short cooldown it can:
Be used to help your team gank effectively,
Spot an enemy gank before it happens,
Allow you to scout Baron/Dragon/buffs risk free
If used at the beginning of the game allows you to see which lanes the enemy are going to in order for you to send suitable counters into the lanes.

Janna Tips

Janna is a support, if you want to play her aggressively this build would probably allow you to do that, but that's not what I'm going to talk about.
When I play Janna I have 2 priorities, keep my lane partner alive, get my lane partner kills.
The minions are not for you! They are for whoever you are supporting, that's why there are 2 gold per 10 items in the build. Unless the minions are on your turret or you partner is out of lane, no touchy!
If your lane is ganked by several enemies you may not have enough mana/CRD/time to slow them all down in time for one of you to get away, in this circumstance one of you is clearly going to die. As a support if all else fails, falling behind your teammate and allowing yourself to be killed gives your ally time to escape. Of course I am not advocating pointless feeding and if there is a way for you both to escape (a Monsoon or a well placed Howling Gale) then obviously take that option, but if it is not available it's better that you get a death than your teammate.

I will only use Clarity if a teammate has low mana, if i have low mana I will use a pot. I will use Monsoon as a heal but again, never for just myself, I much prefer to use it in a team fight when we're getting steamrolled and need the enemy to back off for a second and give my team a heal.

With great AP comes great responsibility! For example, you're laning with Irelia, the enemy Fiora is escaping into the distance, you use a Zephyr to slow her down so Irelia can kill her and... oops, you've got yourself a kill. It happens. Irelia might not be too impressed, because let's face it, everyone treats supports like cr*p, but you can make it up to her later when you save her from a Karthus ulti with your incredible shield.

Advice for First Time Supports

At first, whatever you do, it will be wrong.
If your lane partner has an enemy champ very low but they are very low too, you have 2 choices. 1 you can shield your ally and hope they can kill the enemy before they get killed. 2 kill the enemy before they kill your ally. In the first scenario your ally dies and they start screaming at you because you're a "n00b" who didn't help. In the second scenario you're a dirty kill stealing ****.
People expect to be able to run in half health in against 5 enemies and live because you're supporting them, then they're shocked when they die and of course it's all your fault!
However, over time you will learn to make the right choice, half the time it's done on gut instinct and you'll know when you got it really right because someone will thank you. If someone thanks you it's a big deal. A support is quite rightly expected to be very helpful, you don't thank the AD carry for getting lots of kills do you? So when someone thanks you for your support you know you did well.