Hi! My name used to be Debonair Karma but now it's Wintry! I'm Wintry, I play on EUW and EUNE and I've been playing Karma since around season 2. Playing her mid has helped me reach my highest rank, diamond 4.

Just a heads up - I don't suggest you should play her mid after the -20% ap scaling nerf on her Q (atleast not when you're supposed to be your teams primary source of damage). I basically stopped playing her at all when the nerf happpened. We just gotta wait for the buffs after the champion***** are over.

Pros:
-Easy to pick up
-Safe laning phase
-Useful when behind
-Great teamfighting
-Decent poke
-Disengage
-Can enable engages
-Great survivability

Cons:
-No reliable CC
-Reliant on teammates
-No real powerspike
-Not the strongest damage dealer

You can trade Manaflow Band for Nullifying Orb if the enemy has threatening AP burst. Nimbus Cloak isn't that great when playing mid Karma. Always get Transcendence. Scorch got nerfed and is now really awkward to use, get Gathering Storm instead. For secondary runes preferably go Domination and take Ravenous Hunter and Taste of Blood. You can exchange Ravenous Hunter for Relentless Hunter if you're playing against a champion with low waveclear, it'll make your roams even better. If you go for Resolve (assassin matchups) take Bone Plating and Overgrowth. Revitalize and Font of Life are ok too. You can also go Inspiration but the only viable option there is Insight + Minion Dematerialier for better waveclear.

Luden's Echo
Preffered first item on mid Karma. It offers lots of CDR and mana which is very valuable early on in the game. Also really helps with waveclear.
Morellonomicon
One of the favourite choices for a second item. I wouldn't bother upgrading it past oblivion orb early on though, unless the enemy team has lots of sustain.
Rabadon's Deathcap
One of the best items in the game. Getting it this early on might not seem that great, but it increases Karma's damage output by a ton, also improves her shields by lots.
Void Staff
Cheap item that increases damage output by a lot. Get this right after Rabadon.
Zhonya's Hourglass
Very standard defensive item. It can be purchased earlier in between other items if needed.
Banshee's Veil
Can be bought instead of Zhonya's if needed against hard CC
Ardent Censer
Very valuable item on Karma as it gives her a lot of utility, damage and movement speed.
Definietly get it when your team is lacking an enchanter.
Athene's Unholy Grail
Optional item if you're going for Ardent Censer. Not that important but it's a nice finishing touch.
Rod of Ages
A bit risky item because it doesn't give any cooldown reduction which is pretty important but it scales really well into mid game and helps Karma a lot. Very solid item.
Archangel's Staff
Combined with Rod of Ages it does wonders late game. A build consisting of RoA + Seraphs + Zhonya's Hourglass is incredibly strong and safe.
Liandry's Torment
I only suggest buying it if you don't have anyone else to deal with the tanks.
Spellbinder
Fun item to try in normal games. Combine this with Celerity.
Shurelya's Reverie
Not the best option but if you're not doing too well and your team lacks hard engage get this and prove your worth.
Lich Bane
Again, don't buy this in ranked, but give it a try in normals. Lich Bane is one of my favourite items and it's highly unappreciated.
Rylai's Crystal Scepter
Buying this is pretty much pointless. Only 2 of Karma's spells do damage and one of them is a slow already. Getting it just for the W isn't the smartest choice.
Twin Shadows
There is only one (1) champion in the game that can utilize this item well (Vel'koz). Don't buy it. Please.

Nothing new here. You can pick up W at lvl 2 or 3 and besides that nothing really changes. Sometimes you might want to pass up a point in Q for a point in E though, against champions like Lux or Brand.

The laning phase depends on the matchup.

Against poke heavy / tear mages you want to shove waves under the tower as fast as possible, so the enemy can't farm very well. Ward top river and stick to the left side of the lane. Try poking with Q while clearing waves. If you see an enemy jungler approaching run away immediately, you're not made for 1v2's. If your jungler is approaching, run at your enemy with E and W them while slowing with an RQ.

Against assassins you don't want to get too close to their all in range. Try to avoid any poke they might throw your way because they're just setting up an all in to burst you down.
Waveclear from afar and freeze the wave near your tower so you can safely farm with AA's.
Ask for early ganks and always spam ping when they leave vision. Follow them if you know where they're going.

In all of the sccenarios always play safe and don't give any kills to the enemy.

This is the time when Karma starts scaling. You want to start grouping with your team around objectives, whether it is herald/dragon or just taking towers. Karma is really good at skirmishes and small teamfights even this early on in the game. Try not to throw out your R right away, as you might need it on another spell a few seconds later. Always try to hit more than 1 person with RQ and don't be afraid to use RE instead. Keeping a teammate alive will be more important than dealing a bit more damage yourself. You want to start peeling for your bot laners, stick close to them. Get blue trinket and scout areas so you don't get collapsed on.
Always keep a control ward alive.

At this point in the game, you never want to go anywhere alone. Preferably stick to the bot lane or a jungler like Udyr, Kindred, Jarvan or anyone that deals damage.
You should have most of your build completed, so hitting a carry with RQ will effectively take away more than half of their HP, and your RE shield will block over 1000 damage.
Try to poke your enemies before you fight them.
When you get engaged on or your carry gets engaged on don't hesitate and instathrow an RE. Using your other damaging spells will greatly reduce your R's cooldown anyway so you'll be able to put out tons of damage too.
If your team has no hard engage then it's on you to start teamfights. If you have a tank with CC use your RE to let them engage on their backline. If you don't have a tank and your team is filled with assassins for some reason stay back and spam RQ.
Try to avoid all CC and don't get too close to the enemy front line. Stand back and hit them with Q's.
Using RE on your front line is fine, too.

Karma has an easy time roaming because of her fast waveclear. If you notice the enemy side lanes pushing try to shove real quick and ask your laners to clear vision if possible. Use a few E's on the way to not waste time. When you arrive, always shield an ally to enable an engage. Use your W on the target with less mobility if you're ganking bot.

Also try playing around your jungler. Ask them for their clearing path so you can gank a lane together for an easy 3v1 or 4v2.

Always inform your team you're going to roam ahead of time. Don't forget the enemy laner exists, too.

Blue side wards:

Red side wards:

These are the best ward spots for both teams. All of the dots in bushes are also the best control ward placements.

Karma might not seem as a great pick right now (judging by her 48% win rate at the moment) but she is highly unappreciated. If played well, Karma suddenly becomes a versatile mage with just the right mix of damage and peel. Pick her if your team is lacking utility. The package comes with free burst damage. Isn't that fun?