When trading use you E first so you get cooldown back ASAP. Hitting your W , Q or knocking enemies back with ult refunds some cooldowb off your E, thats why you use it first, second i recommens Q, its also good for poking. If its a short trade rry to use as many basic attacks as possible before casting W. W gives you movement speed and your passive gives you bonus magic damage based on movement speed so try to make the most damage out of it.

Bad point is that Janna doesn't have that much base damage like other marksmans so she scales a bit less but it will be fine if you win early game and she has a lot early game damage. Try to plat aggressively, especially if enemy adc is someone like Vayne or KogMaw.

1.Your Movement speed form W, it makes kiting a lot easier to execute.
2. Q self peel, if enemy assassin jumps on you, you can knock him up with Q and give yourself some time to get away from him. You might even interupt his jump/dash.
3. Marksmans usually dont have shields so they are easy to kill but guess what? Janna has one, that makes you harder to kill.
4. Janna's ult is great dissengag ability, bruisers and assassins hate it, use it to pell yourself or your teammates.

This guide is created mainly for fun so maybe shouldn't play Janna ADC in ranked unless you are really confident that you can win.