Hello , My name is Sara. i main a lot of Supports Champions. I'm sorry if my English is bad , because English isn't my mother tongue. This is my first guide, hope you like it.

At first your main path will be sorcery since you will go half AP. I picked Arcane Comet because it can deal damage if you hit your W. It will always hit the ADC , because Your W slows him down so 99% it will deal damage to him.
Then i picked Manaflow Band , in early game you lose a lot of mana hitting the ADC OR SUPP . so you need it to restore some of your missing mana. At last i chose Celerity and Gathering storm , because you are toxic when you go AP. your W is Really annoying and can be toxic it slows down enemies and do a high amount of damage.

The second path is Resolve. I know a lot of you use it when they play only melee champion, but it can work very will with u .

Passive : Your passive is very useful , Janna is almost the fastest Supp in LOL . Try to walk in front of your ADC to give them Movement speed. Your movement speed helps a lot in escaping or chasing to slow them down.

Q : Your Q is very useful against tank supports that have stuns. Also it can slow down the enemies while you help someone to escape. At last it can stops MF ult , Alistar W , Rakan W.

W : Your W can deal a lot of damage even in early game. If you go AP . It really becomes toxic , I've tried it many times in ranked and it really helped us to win the game. i killed 2 full hp champs alone with it. Also it can slow down the enemy team , it will help you to get to them faster or to escape from them.

E : not much to say , but it help and protect your ADC and The Tower.

R : Janna Ult is amazing , it stops some champs Ult like MF. It heals your ADC. It helps in escaping from the enemy team by pushing them away a little Or sending them to the tower so it helps in killing the enemy team. If you notice that the enemy team is focusing you while you're doing the Ult stop it and Use redemption to heal your team. Then use your Q to stop them for a while.