This build is designed to allow Janna to take full advantage of the new Healing & Shielding bonuses that have been added to a few items in the game. Below are all the bonuses, along with other Healing & Shielding buffs that you'll be getting from masteries.

The idea behind the chosen items is simple, and it's to take advantage of just how much we can buff Janna's shielding and even her healing from her ult. Before I get into explaining the items, I'll lay out some numbers to demonstrate the bonuses.

Windspeaker's Blessing / 10% Bonus
Runic Armor / 8% Bonus
Mikael's Blessing / 15% Bonus
Ardent Censer / 15% Bonus
Forbidden Idol / 10% Bonus

All of the buffs from the items, along with the Windspeaker's Blessing Keystone stack. While anyone using the Runic Armor mastery or owning a Spirit Visage will actually have that bonus applied on top of the ones that stack. This adds up to some pretty decent numbers if all the buffs fall into place.

The Numbers

Below I've listed a few of the different overall bonuses for several different combinations of bonuses, along with brief explanations of scenarios in which you can use them in different ways.

Windspeaker's Blessing x Runic Armor = 18.8% (48.5% with Spirit Visage)
Something to consider here is that this will be what you start off with in each game. But for the early/mid game, this will benefit you more than your ADC, as your ADC won't likely have the Runic Armor mastery, meaning your ADC will only get the 10% bonus from Windspeaker's Blessing .

Windspeaker's Blessing + Mikael's Blessing = 25% x Runic Armor = 35% (68.75% with Spirit Visage)
When it comes to getting the Mikael's Blessing the bonus shielding to your ADC more than doubles. While it does not increase as much overall for those who are using Runic Armor , but the benefit your ADC gets is really noticeable.

Windspeaker's Blessing + Mikael's Blessing + Ardent Censer = 40% x Runic Armor = 51.2% (89% with Spirit Visage)
Once the build gets to this point, you should also have locket. The total bonus Healing & Shielding make Janna a nightmare in a teamfight with all the Healing from her ult, the Shield from locket which applies to the whole team the 15% Attack speed, and the 30 Magic Damage per hit buff provided by Ardent Censer, and all of that on top of the shielding from Eye of the Storm.

Janna's skills sequence doesn't have much variety, except for at levels 2 and 3. Below are the two different options with an explanation of their use.

Here we start off with Eye of the Storm as usual, but we take her W Zephyr second for the passive move speed it provides. Think of having boots almost immediately in lane. This move speed allows you to gain more control over the lane by out positioning your opponents, use your shield and the move speed to gain better control of the lane brushes, and zone the enemy support while not taking bad trades. However, this only works into lanes that are not able to jump right on you and kill you, so this doesn't work well at level 2 into lanes with heavy engage.

Taking Howling Gale at level 2 works great into lanes that will be looking to engage at level 2. Take Howling Gale second in order to deny champions like Leona and Thresh early on in the lane.

For the most part the Masteries are generic on Janna. However there are some things that I would like to explain, including how our Runes and Masteries compliment our build.

We get a total of 10% CDR from our Runes and Masteries, which allows us to get to 30% CDR really early on in the game by buying Ionian Boots of Lucidity and Eye of the Watchers.

We get this 10% CDR from two sources. We get 5% from the 6x Greater Glyph of Cooldown Reduction Runes, while we get the other 5% from the Intelligence Mastery.

Of course the other noteworthy Masteries are:
Windspeaker's Blessing
Runic Armor

Janna is a champion that roams extremely well thanks to all the extra move speed she gets. Once you start to max her W Zephyr, you'll be able to get around to map really quick. You'll want to use this to your advantage by securing vision and also assisting other lanes where you can. Aim to jump on really small windows, like seeing their Jungler top side, or having the enemy bot lane recall gives you plenty of time to move into their jungle and secure valuable deep vision without ever having to worry, thanks to the move speed.