This is my first guide of Janna I don't know that will it works or not but i played and it does work.

Overview.

Janna have a very high ap damage with the [Howling Gale] but this is hard to aim to the enemy because it was a channeling skill. (Enemy saw the whirlwind on the ground they will be dodging the [Howling Gale] and you can't damage them. But I have a trick that you will have more percentage to blow them up.

Pros

Can harass often without giving up CS
Her shield can prevent her from taking any damage while harassing
Her Howling Gale can wipe an entire wave of minions
Her passive AND Zephyr give her speed buffs along with being able to pass through minions
Can sustain in lane for long periods of time with clarity and her ultimate
Still works beautifully to support in team fights
Hitting Q twice quickly can interrupt certain ultimates and other channel abilities
Your mid turret WON'T go down


Cons

Requires blue buff to be fully effective (mana hungry)
Squishy
CC can pick her apart quickly
Cannot mid against anyone with a silence
Only has 2 damaging abilities

First of all get Doran's Ring, this item will make you a little bit tankier and regen your mana (5 mana per 5 second). Just use your Howling Gale in the lane aiming to the minion for money of to the enemy champion to harassing them from farming the minion. That's the only way you will get the money for it. When you got like 1200 - 1900 you can return back to your base to buy Needlessly Large sword and boot if you want. (Its up to the situation).


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Repeat these step and brought the item that i recommended you. That all...

Aiming techniques

Random tornado - Aim the tornado randomly each time. This way your enemies cant actually predict where it will go and dodge it by doing the same thing.
Invisible tornado - Cast the tornado in the bush so your opponents cant see it. That way they might get caught without even knowing you are casting it.
Pushing tornado - Although its lvl 1 it can do severe damage to creeps and it can result pushing if you aim it at creeps only. Other point is that some players uncontiously hide behind creeps and they might get caught. Combine it with the other Aiming techniques and the enemies might get caught cause they wont know that you aim for creeps.
Reverse tornado - If you are being chased or you see that your enemies want to dive you just aim the tornado in reverse direction (towars you). Most of the enemies actually thing that if they pass trough the growing tornato before released they will avoid being caught but they will get a bad surprise and you will have enough time to save yourself or even harras, slow or kill them sometimes if they are low.

Interrupting tornado
1.If there are enemies with channeling spells ( Nunu, Katarina and etc) always be sure you have the tornado aimed in the middle of the fight cause they will most likely position there if they wanna channel their devastating spells. When you see they are channeling release the tornado immidiatelly.
2.This kind of hard because you need to time the release very precisely, but when you get used to it you will have great advantage. It is actually interrupting the charge/leap of melee champions while they "fly" towars you or your ally. You must release the tornado in the exact same time they charge/leap and it will result interrupting and disabling it.


Credit from: Panglot
This is a great trick that i have follow so i post it here again..