First Rule every Supp has to learn is not to deny farm for the adc.
That might be easy to say but really hard to do if the adc is brainafk. As supp you should try to zone enemies from the wave to deny them farm but if that doesnt work yet you are allowed to push without last hitting.
Always push the minions under the enemies turret if you decided to go back, therefor you may get some lasthits if the adc is not pushing far enough. That will deny the enemy's adc a lot of farm, through to the faster waveclear the turret is giving.
If your wave gets pushed close to your turret, but not in its range, you are not allowed to do anything to the minions, that is all your adcs job and he will thank you that. You may only tank some of the minionhits if the minions of your enemy would otherwise run into your towers range. This has its own name "freezing" and all adcs love if their wave is freezing cause they will get maximum advantage out of the wave without getting in danger of being ganked.

If you try to poke the enemy what will be your main job with Janna, you need to learn to do that safe and succesfully. Therefor you need to learn that most adcs will run some steps back each time they attacked a minion previously. For you this is the time to shoot your Zephyr and a basic attack. Therefor you have some bonus movement speed with your passive and a shield to block incoming dmg and get the positive trade.
Keep sure the enemies supporter will not use this moment to hurt you with their cc. It would kill you instantly.
If you fear them too much just trade with them at first. both adcs will stay far away enough to not get punished, so that you can simply trade in 1v1 which will be a good trade for you in most cases, because your shield blocks almost each spell completely and will buff your ad so your basic attack that follows your zephyr will hurt more than before. in case you play against hard engage champs you should try to place yourself between minions, that might block hooks or grabs, or be ready to press your q twice as fast as possible to deny them the gapcloser.

if your wards spot an enemy roaming or ganking you, keep sure to spamm-ping your adc and place yourself between your adc and the roaming player. In case he has a gapcloser, keep your q ready to block it. In case you miss that one, use ult to kick the enemy backwards.
If the enemy is Yi or someone similar without a real gapcloser but the possibility to run you down, use your q on the enemy to get some time to run away. If its not Yi nor Olaf you should turn from time to time to give them the slow of Zephyr.

keep the river warded and always keep a pinkward with you. the moment the jungler reaches the vision of your enemies, slow the adc and be ready to shield the one that needs it most. if you have the advantage of being 3v2/3v1 you may use your q to stop their disengage if possible. Tank as much tower shots as you can do without dying by guarantee before giving the aggro to your teampartners, in case you dive them.