It's really hard to cover everything and there is so so so many possible scenarios but I will try my best to keep it simple and to cover the basics and most crucial things that you can apply pretty much every game.
Starting off with laning phase:
Level 1 you start Zephyr and try to get prio. Attack the minions to make sure you hit level 2 first and play around the bushes. Ideally you wanna be going in and out of them, using Zephyr on the enemy and going back into the bush before they can trade back. Don't forget the enemy jungler though, and if needed (because you don't know where he started, or know he will be bot), go ward after the 2nd wave.
Try to punish the enemies for stepping up for minions and try to stay next to your adc. Don't walk up too far by yourself while he's farming. Generally, you want to try to recall at the same time.
You don't really want to be spamming Howling Gale, it costs a lot of mana. It's good to use it here and there, walking out of vision to cast one or placing it in the bush (make sure you don't leave the bush until it goes off, as long as Janna is out of vision, Howling Gale is invisible too.)
As the game goes on, you want to be looking for fully charged Howling Gales from out of vision. Each Howling Gale you hit might grant you a kill. A great position to place it at is the enemy's auto attack range (where they stand while hitting minions or the turret) it has a high chance of landing.
 | A great trick with Howling Gale in lane is to aim it like you see in the picture, it's undodgeable because of the turret and enemies almost always get hit by it. (arrow depending on your position) |
| The enemy jungler is ganking! A very common mistake is that Janna players try to hit the jungler with Howling Gale while fleeing. You should however place it as depicted in the picture, cutting off his path. This way he can never reach you unless he flashes it. This applies to literally every single scenario where you are running from someone. Use Howling Gale to cut off their path, don't place it trying to hit them in their current position as that is easily dodgeable. |
Roaming:
Make sure to always look at the state of the wave. If there is a big wave about to crash into your turret, you need to always be there with your adc, or they might (and most likely will!) get dove. You can never leave in that situation.
| You generally want to walk back following the arrows you see in the picture. This way you can decide wether you just walk back botlane or need to be mid because something might happen. |
| If you just crashed a huge wave (the circle) that is still under the enemy turret, you have a bigger timeframe to roam. In that case, you can just straight up walk mid, or maybe even toplane! See what your jungler is doing, maybe you guys can invade or make a gank happen. |
| If you and your adc just crashed the wave, you have a small timeframe to go do something. You could go ward enemy jungle (but be careful!), invade it with your jungler, or see if you can maybe make something happen midlane. Make sure you are back in lane by the time the new wave is there so your adc can safely farm it. Generally, every time you crash a wave like that you want to go in the fog, so enemy doesn't know where you are. When you come back after the roam, stay in the fog too. Maybe the enemy botlane thinks your adc is alone and engages not knowing you're there too. 2 free kills! |
Mid-game:
This can get pretty advanced and kinda depends on how the game is going and is mostly just macro and not very Janna specific so I'm just going to keep it simple and go with a few important bullet points.
The first thing I'd like to mention is kinda random but I haven't really been able to put it anywhere. Always use Eye Of The Storm on your ally before doing anything else, because whenever you impair an enemy's movement with Zephyr or Howling Gale, 20% of the cooldown is refunded. Also make sure to remember that you can cancel lots of things with Howling Gale and Monsoon, so it can be good to save it in teamfights! You can cancel for example Death Lotus, Elastic Slingshot, Ferromancy - Crash Down / Mount Up and so much more!
Now actually onto the mid-game.
At this point, you want to be spamming Howling Gale on the waves whenever enemies are there. You don't lose anything from it, and one might hit and grant a pick.
Play with your jungler once the mid-game starts. Help your adc catch the wave mid and once they are safe again and waiting for the next one, look to do something with your jungler until the next wave comes in.
Play on the side where your jungler/carry is. If they are in the top jungle and you and your adc are mid, make sure you are on the left side of mid.
You don't facecheck at this point, unless you know where everyone is. You can use a fully charged Howling Gale on a bush as you walk towards it to keep you safe while you ward.
Ping your team. This is super important! Do something useful with the kills you get. Did you just kill enemy jungler and baron is up? Ping them to get it! Don't keep aram'ing mid.
Stay behind your team, we aren't playing Alistar and we can't save anyone when we're dead ourselves.
Make sure you always have a few wards, and recall in time to get some new ones if an objective is spawning. Don't spam-buy Control Wards, only get them for objectives.
You can use Monsoon to push the enemy jungler away from an objective as it's about to die, so they cannot Smite it.
