Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
In the early game you will be using you q to harass and prevent the other adc from farming while using your e to shield your adc from damage. Janna is good at keeping her adc alive so make sure your adc stays alive and try to stay behind him when he is running away and in front of him when he is going in for a kill.
In the mid game you will want to group with your team after you use the banner of command on a siege minion. if they go to deal with that lane you can force a fight and take mid turret or another objective and if they ignore the bannered lane then it will take the tower.
You will want to set up the banner and the zzrot portal in any lane and then use them to push. youll almost always be able to take a tower and with your w plus items youll have so much movement speed that no one can chase you down. just be careful that you dont get collapsed on. try to go join your team after you set up your push. if neither team is ready for a teamfight though try to stay and take a turret or inhib then run when the enemy team comes to kill you.