The first thing i want to say is that Karma is a very good 1v1 champ,so she can handle mid very well and is able to do a quick first blood at lvl 3.Most of players underestimate Karma,so this could be an advantaje cause when you are low hp you have more ap,they are tented to attack you,so now u will kill your opponet by using Mantra+Soul Shield then Heavenly Wave and Ignite.The shield will protect you so you won't die.When you are mid asure that you will kill as many minions as possible 'cause you can reach 4.5k gold very fast without dying or recalling.Use your Heavenly Wave to kill minions and harras your enemy

More AP to kill your opponet or to save yourself.

Doran's Ring for hp and mana recovery.
Deathfire Grasp for CDR and damage so you can kill your opponet even faster.
Ionian Boots of Lucidity to complete your 40% CDR so your Mantra will have 18 seconds cooldown.
Abyssal Scepter.When your enemy will see that you can kill them so easy,they will take some magic resistance,and you will counter their defense.

Heavenly Wave-good damage+heal.
Why Soul Shield instead of Spirit Bond?Because you can slow them with Spirit BOnd but then you will have just Heavenly Wave so you won't be able to kill them.Instead,Soul Shield is good for defense and offense,cause by using Mantra you do damage equal to your shield protection,and you can kill the enemy very fast with Heavenly Wave and Ignite.

This build works very well for an AP carry Karma,that's why you need to take mid lane and to kill your enemy as much times as you can.
I hope this build will help you to understand Karma better ,'cause she is very underestimated but, actually, she is very powerful.