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.
First i would like to say that Janna is an amazing solo laner in the early game, and can handle herself against 2 people fine if you have a jungler.
Doran's Ring is a good item for most and you need the health, otherwise i would say go dagger if you thing otherwise. The health pot is mostly because you have clarity and it should hold you down until your ult so you don't need to go back.
When you do go back, You should try for your boots and the stinger.(seeing as you can stay in lane for a long while) If you can't stay that long try until 1k for boots.
Finish Nashor's at this time you should be around the 20 minute mark, give or take a few minutes. After that start and finish your rage blade, the last 2 items are more or less just gravy, and arent needed for most games, but i would suggest you get Rylai's before Zhonya's just because of the health and the aoe slow your nado gives after you get it.
Ap stacks with all of Janna's spells and is always nice for damage output. And as for why the per level? Well Janna doesn't need to any Ap early game to get kills or to be effective, so this paired with blue gives you half an items worth of Ap, which is quite good late game.
In combo with your Mastery Tree and your Nashor's you should get something like 30% cd, and the max is 40%.
Skills and Tips to come.