This is going to be a big post, with quite a lot of criticism, but don't take it personally or see it is as an attempt to slash your guide. It's meant to give you tips on how to better play Janna, so you can improve your own gameplay as well. :)

Cheat Sheet:

• As Emoriam already mentioned, apart from items and summoner spells, you can also list runes, masteries and ability sequence
• It might be a good idea to add small notes to the build paths, so people when they should get what item and for what reason etc.
• There is no mentioning of trinkets whatsoever, which are, especially supports, rather important
• You could consider swapping the images of Health Potions for that of Total Biscuit of Rejuvenation

Pros & Cons:

• I would swap the coding so that all the pros and cons are just plain white, and the titles "Pros" and "Cons" are green and red instead. This looks a little weird
• I would not say Janna has okay poke. Her auto-attacks deal next to no damage, and her tornadoes and slows shouldn't be used as poke, in my opinion. Your laning phase is mainly about just defending your carry

Runes:

• Gold quintessences are soooooo season 3 (*mimics girly girl*). You are way better off with health quintessences, as since the preseason supports already earn more than enough money
• Seeing as you get armor in marks (and if you consider my suggestions, also quintessences), you can better back that up with health in seals than armor

Masteries:

• You could consider swapping your masteries to 0/16/14, because otherwise you go over the cooldown reduction cap. Another possibility is going 4/5/21, and leaving out one 10% cooldown reduction item
• The way you spec'd your points is perfect, well done!

Abilities:

• Skilling sequence is right, good job
• You could add more in-depth explanation on how to use Janna's abilities, and more little tricks because that's what we want to know if somebody is teaching us about Janna :^)

Summoner Spells:

• Generally spoken, there are only 4 viable summoner spells for supports, being Flash, Exhaust, Ignite and Heal. The others are just plain bad, or outclassed by the aforementioned possibilities

Lore:

• I would not include lore in a guide, but that's just my opinion. If you do put it in, consider moving it to the introduction chapter and putting it in spoilers

Gameplay:

• I would not shield the jungler (might be because I'm a little selfish person :3). Most junglers can easily clear without the shield anyway and you only end up wasting mana
• You're talking mainly about shielding, which is of course good and an important part of Janna, but the rest of her kit deserves more recognition. Janna is very good at peeling and disengaging, which is actually the main reason you pick her

Adivce & Tips:

• Like you said, you can miss your tornadoes, they cost a lot of mana, and they deal just little damage - you should never use it to be poking. It is a one point wonder, and its best use is tornado'ing people away from your AD carry. You should generally always save this spell for the occasion a Leona decides to Zenith Blade your AD carry, or Jarvan IV comes flagjumping in

Good beginning of the guide, you clearly have an idea on how to play Janna. There are quite some points of improvement though. I will not vote for now, but if you decide to adapt your guide to my review, feel free to send me a personal message and I will have a look again. :)

Thank you for your feedback!

Next to items you show Runes, Masteries and ability sequence in the cheat sheet as well.

I don't really think Ardent Censer is good enough as an item overall to warrant it as core.