"""Edit: I just realized you were asking about this from the adc's perspective lol. Well at least you know now what goes through my mind on how to win the lane when I play a squishy support."""
Plat 5 Support main here. I just literally finished a game playing janna with my teammate cait vs a leona and ashe bot lane. We won the game and got a double kill in lane around 12 minutes into the game.
Here's how you lane with janna vs a leona. So as I went into this game I told cait to play very aggressive lvl 1 because leona's usually start q (although they don't always). However, leona is not very strong lvl 1 and you can out trade them at lvl 1 with your janna shield as long as you don't use it unnecessarily. Then leona has to burn through some pots. Go into the enemy bush and start autoing them then going back into the bush. Force them to use their trinket to ward the bush. (btw you should have ad reds imo which help a lot with this).
With janna you won't always win your lane, but you sure as hell shouldn't lose it very much. If you get ganked post lvl 6 and leona e's in and ults don't flash away like a bitch and run. Don't run but instead Ult her into your tower so your adc gets a free kill, or at least blow her flash. If the leona ever hops on your adc in lane shield your adc and use your tornado to knock up the enemy adc. Most of the time you won't have fast enough reactions to outplay the enemy leona's e.
In the mid to late game where you group up etc. Always sit in the back. Shield the tank if he is initiating or shield your adc if he gets jumped on.
IMPORTANT: if your adc gets jumped on rapidly and you or him are about to get instakilled butt fk'd. ULT ULT ULT ULT ULT.
Janna is very good at resetting fights. If you see you are gonna lose a fight cuz your only dmg dealer is gonna die. Use you ult to say nope enemy team not today. Don't feel like you need to ult randomly in a fight if you are winning the fight. Lots of times that leads to letting enemies get away. Lastly, janna is a really good peeler so shield your dmg dealer and w whoever jumps on your adc. Then tornado them etc. Sometimes you should buy mikaels crucible because the active is really helpful. You just gotta learn to use the active well.
Lastly janna is pretty op at the moment and a great pick. Just learn to play her disengage peel play style. sorry for the bad grammar and spelling but the knowledge and info is there. If you have any questions let me know. Peace