This is my first guide and I will be updating it every patch. First off Kennen is an amazing Champ and is very underplayed. I noticed that there are not many good Kennen guides so I thought I would make one. This is my build for Kennen and if you have any helpful changes feel free to post them in the comments :)

For Runes I go with 9 Magic Pen marks, 9 HP by level Seals, and 9 AP by level Glyphs. The reason I use seals that scale up by level is for better solo laning. Finally I use 3 Flat AP Quints. This is what works for me but feel free to swap out for whatever you think works best.

Masteries are fairly obvious I go 21/9/0 for extra damage and AP and I put 9 points in the Defensive tree for much needed mr and armor and now I start off with 5 hp bars so if I decide to buy boots instead of a doran shield then I won't look so squishy and it's just a whole lot safer ,late game you will be focused lots extra tankyness helps quite a lot. You could also put 8 points in the Utility mastery tree for some movement speed with 10% less time dead if you wanted to. If you are new to Kennen then I would recommend going in the Defensive mastery tree 9/21/0 for extra AP Spell Pen from the Offensive tree and then getting all the major defensive buffs in the defensive tree.

OR Staring with boots and 3 hp pots is great depending who your laning against if your new to kennen grab the Doran's shield. I almost always go with this exact build these days, it works well for me and I do consistently good every match. Rushing Deathcap right after boots works if your doing good but this is just more of a realistic build. I'll typically buy my Sorc boots before Will of Ancients and then I'll grab a Blasting ward or if I'm doing really well I'll just grab a Large Rod. If your doing REALLLLLLLLLLLY good and the enemies aren't interested in murdering you, then getting a Soulstealer can be very good sometimes, other times it may just get you focused. I get the Rylai's crystal scepter for it's amazing passive which is great for chasing people with your Q also it makes it easier to hit everyone with your ult. If you every only have like 300 - 400 gold and your item is far away then Elixir of Brilliance is amazing. If the game goes on to long then sell Will of the ancients and buy Lichbane or Deathfire Grasp depending on the enemies, Deathfire grasp will nuke tanks really hard, Lichbane is more of a luxury item and it will destroy turrets insanely fast. Getting a Void staff isn't always necessary if your enemies aren't building mr though 8/10 games they will but it will still give a slight damage boost on everyone.

I buy sorc shoes almost every single match. Sometimes getting merc's treads will be more useful, for example if you laning vs Veigar, or maybe they have a lot of cc but no MR.

Deathcap is the best item a ap caster can have.

With hourglass your invincible for 2 seconds which is amazing combined with Kennen's ultimate.

WoA is a great item, a few Q's and your full hp. It also helps your team out with it's aura effect.

These are pretty much the only items you'll need to do well with Kennen.

Pros
+Great Solo Lane
+Great damage
+Invaluable during a teamfight
+Great movement speed
+Great farming
+Realistic item build
+Consistently good every match
+Your a Flippin Ninja


Cons
-Squishy
-Get's focused if your enemies have brains
-CC murder's you and can make your ult useless (Vanye shooting you away, Lee kicking you out, etc..)

Kennen's passive is what makes him a good champion, hit an enemy 3 times with any of your skills and their stunned, pretty simple really.

I max Q right away, it's Kennen's main damage output and harassment, being good with this skill is what makes a Kennen player. You won't hit everytime but being a good with this skill shot will pick up kills and guarantee stuns.

I max Kennen's W next because you can have insane Harassment combined with your Q. Kennen can shut people off from their minion waves pretty easily with Q,W combo.

Kennen's E is a great farming skill and also makes it super easy to escape from ganks but it just has to much energy cost for teamfighting which is why I max it last. I'll talk more about Farming in the Farming section. Make sure if you need to get somewhere fast then E there don't just walk. If you use E to chase somebody make sure you hit a minion so your energy is replenished. I pick up E at level 2 to escape from ganks and such.

And finally, Kennen's Ultimate, this is what makes Kennen invaluable in a teamfight. At level 16 your Ultimate will deal huge damage to everyone and stun everyone once if it hits them 3 times, the trick is hitting everyone 3 times with it. I use E for insane movement speed then R and hit as many people as you can, should be pretty easy while E is active.

Spam this as much as you can, it has a super low energy cost and is Kennen's main damage output.

After you tag them with your shuriken then zap them with this.

This is a good skill if their bunched up and then you can just run through them all and back out then hit W and zap them all then Q into them.

Use this to secure kills or hammer everyone in a teamfight. If you have lots of energy (75%+) then E first and hit as many champions as you can as well as having R active at the same time.

For Summoner Spells I grab Flash because it's one of my favorite spells and Ignite because it's great for picking up early game kills. I've used Cleanse and Ghost both of which are great spells and work good on Kennen, Cleanse especially because CC destroys Kennen but I prefer Flash and Ignite. Some spells I would NOT go with on Kennen are - Clairvoyance, Resurrect, Clarity (obviously), or Smite. The rest of the spells are decent enough.

Kennen is an amazing Farmer and can easily have the most CS in the game if your good at it. Early game farming is VERY easy thanks to Kennen's Q, and Mid/Late game is even easier using Kennen's E then W to take out an entire minion wave. Early game all you need to do is last hit with auto attack and then Q when you don't think you'll be able to last hit with your auto.

Kennen is the type of Champ that can win a teamfight. If a fight breaks out then you can flash right in the middle or it hit R then hourglass and watch the show. If there is just a few enemy's then Lightning Rush into them and hit R then W then Q to last hit them. If your in a teamfight and you don't have your Ulti then just Q them and if they run then chase with E and Q then W to finish them.

Thanks for checking out my Kennen guide! Feel free to leave constructive feedback and any tips or tricks that you use.
I'll keep the guide updated as much as I can in the future :D

PS: The images have been bugging out lately sorry it makes things a little more complicated I'm working to fix them all.