Your main focus with this build will be basically be doing nothing but peeling and buffing your ADC by spamming your E a lot. I go over the abilities, and some tips on how to use them in the "abilities" thing. Don't be aggressive on your own and wait for your ADC to initiate then back them up with shields, and slows on the enemy, but remember your most important job has to be never letting your ADC die which is why you need to save your Q for disengaging instead of harassing the enemy. Leave the harassing up to your ADC and just watch their back.

Heal and Exhaust are just a good set of spells for any support out there no matter who they are. Also supports who use ignite piss me off.
Heal. well heal is almost necessary for Janna since you don't have any solid healing other than your ultimate, and extra move speed never hurt anyone.
Exhaust. I honestly have no idea why every support that ever plays the support role does not take exhaust. Exhaust is such a good spell in so many ways. the slow alone is a good enough reason to take it but if you pop it on the enemy ADC right before you and your ADC hard dive them for a kill then your ADC is almost always guaranteed to win due to the 10 reduced armor and MR, 30% attack speed and walk speed debuff, and a 40% damage reduction applied to the target!!!
40%!!!!!!!!!!!!! that's a lot!
You can also take flash if you really feel that you need to but you shouldn't because Janna is the queen of disengaging. Also you have heal.

Rylai's is a pretty solid item especially if you find yourself dying a lot more than you would like to, but I would lean more towards Deathfire Grasp if you have a lot of magic damage coming from your carries to help buff your carries even more with the active since it boosts ALL magic damage on the target not just magic damage dealt by you.

Q.(Howling Gale) Is mostly only here to interrupt abilities, and aid with escapes. Due to its large use of Mana I suggest using this very smartly, not spamming it.
W.(Zephyr) Is Very good for helping your super buffed ADC stick to enemies and make them cry, Also if used wisely can help land your Q's more often.
E.(Eye of the Storm) Is great and everything else sucks. Please spam it, Best if used when your ADC is about to trade with the enemy ADC since the shield will negate most of if not all of the damage done to your ADC and will give your ADC bonus AD letting them walk away with full health while the enemy ADC limps away having taken lots of damage.
R.(Monsoon) Is a very simple ability and not that hard to use. remember you can use it just to knock away enemies, but I suggest trying to position yourself where when you pop it it will only put your allies in the area for healing without giving the enemy a chance to escape by pushing them so safety. ALSO PLEASE REMEMBER IT CHANNELS FOR A VERY LONG TIME AND THAT IS A LOT OF HEALING PLEASE STOP WASTING IT.

These masteries really cover everything you will need from masteries for Janna.
Lots of sustain, lots of free gold, move speed, And even some CDR for you and your actives.
The few points in offense just to help maximize the damage your carries do because we all know we love to make our carries feel like they are just the best in the world.

The runes are pretty simple the armor Marks are to make it a bit easier for you to stay with your ADC if they are super aggressive.
Mana regen Seals because honestly what support likes to be out of mana.
flat mana Glyphs for even more mana for the skill spam.
GP10 quints because you're a support and supports like to get free money.(I personally prefer EXP Quints for the sheer usefulness of them.)