This is my first build and I hope it will serve you good. I'm not pro at Karma and i guess i will never be but I think this is how you should player her and what you should do to make her usefull. Of course do not take this guid as a "YOU MUST FOLLOW IT LIKE THIS! OR ELSE YOU'RE NOOB"

This is just how I play her and what I think works best.

Teleport: Is good for eveything "OH NO TURRET IS DEAD" *TELEPORT* "NO FEAR KARMA'S HERE!"
Flash: Escape and chasing spell is also a great summoner spell.

- other summoner spell you can use -
Ghost: instead of flash.
Ignite: if you want to get more kills this is good.
Exhaust: also good if you need more kills or an escape spell.
clarity: if you wanna spam spells use this.
Heal: is also good if you're not so good.
Cleanse: other team has massive cc use this

- summoner spells not to get -
revive: WHY!?
Smite: you are not a jungler.
Fortify: let the tank handle this one.
Rally: useless since nerf
clarcoyance: not that great for you. let supports get this.

for runes I choose to go with some simple

Greater Mark of Insight (+0.95 magic pen)
Greater Seal of Clarity (0.065 mp/5 per level 1.17 mp/5 at level 18)
Greater Glyph of Potency (0.99 ability power)
Greater Quintessence of Potency (4.95 ability power)

I know mp/5 Runes got nerfed this patch but I still think they're are worth getting.

I Choose Ap items because it maximises karma's abilities.

Explanation:

Boots: cooldown reduction is always good!
Rod Of Ages: You are quite squishy so a Rod of ages gives you enough hp and ap to survive.
Rylai's Crystal Scepter: slow on all abilities and extra hp makes you incredibly good!
Rabadon's Deathcap: Extra ap great with your passive!
void staff: magic pen for extra damage
banshee's veil: it makes you less squishy and I always use this on any caster I play.

Early Game: you wanna be a bit passive. And wait till around lvl 6-7 so you got enouhg mp regen to heal and damage without worrying about going oom. try to harass then when they step to close use spirit bond to slow down, Mantra then soul shield and finish them off with heavenly wave.

Mid Game: Try to create teamfights because there is when you will shine. Try to not go alone even though you got your rod of ages and maybe your crystal scepter you are very vulnerable but in teamfights you SHINE. Someone initiates, use mantra and Soul shield on the initator. Then spirit bond and try to mova around so everyone takes damage and is slowed, Then use heavenly wave with Mantra so you can heal your party and damage enemies.

Late Game: At this point the enemies shouldve realised that they should focus you. So try to stay back and when the teamfights is raging you come in and do your thing. If you can take golem buff alot. The CDR is really helpful because you can always have Mantra. Dont be afraid if you find someone running around alone in forest. Use Mantra soul shield, soul bound and Heavenly wave combo and they should try to flee or lay dead on the ground.