Hello. My name is PraveBie from Egypt and I am here to help u learning Janna support.
Gold 2 Main janna over 300k points went from Silver 3 to Gold 2 with Janna (over 300 games with her) Eune.
My first time doing guide for chmapions.
Read carefully.

The runes i made is my favorite runes build to me. Since janna consume alot of mana at all game phases yellow Scaling Mana Regen Runes would help her surviving and harassing more with skills.
you need AP runes to scale better on your abilities.
You can get movement speed Quints and AP Glyphs if you like roaming more than playing safe.
And of course Armor and Magic resist for more sustain.

Getting 6/18/6 is the best choice for Janna in my opinion to let your teammates deal 3% increased from Expose weakness mastery. some get 0/18/12 if they need to heal and shield Themselves and for more cooldown reductions on summoner's spells and that's actually very good for janna too, It's your decision at last.

Try to rush getting 45% CDR and Mana Regen to use your skills more often.
you can find more information in Notes in item section, Please read it.

Starting with E to help your jungler to clear faster and survive with more HP, helping your adc at farming with increased attack damage. At level 2 it's your decision to get either W or Q, But i prefer starting with q if enemy botlane trying to jump on your adc and protect him.

In Lane, *try to shield your adc every time they try to poke him down and give him opportunity to poke them back and win the trade. ALWAYS when you gonna use your ult try to place yourself in a bush or a secure place away from the enemy team so they don't CC you and stop your ult.
*try not to use your w if you expect them to start a fight or their jungler gonna gank you so you can have enough speed to run and secure your adc.
*try to place your q in the Fog of war or a bush so they can't see it and you successfully hit it, Janna's Q needs training, don't forget to train on her in Normal matches only, don't *try to train on Ranked matches cause it will end bad for you.
*if someone trying to use a mobility ability (jump like tristana or dash like gragas/sejuani/leona) immediately use your q to cancel it or use your ult to keep space for your adc to run away from danger.

In Roaming. try to take advantage of your passive to roam a lot between mid and river and try to save some lives.

In Teamfights. Try to shield the Squishiest ones in your team, If you have an opportunity to divide them up try to do Flash+Ult so you can push the adc to your team so he can't run away and get more winning rate in teamfights since they won't have adc in the teamfight.

Pros : -Peeling for adc in easiest ways possible with Q-R
-Got Supportive Abilities to help the whole team in Teamfights
-Got CC
Cons : I can't spot cons in her but all i can say she is squishy and got no mobility if she uses her w

Thanks for reading my very first guide for my favorite champion, janna.
hope you learnt new things from my guide and don't forget to share the guide if you like it.
on my next guide i will try to put more details. :)
Sorry for my Bad english.