First guide so please don't hate to much!
And I know doing this on a karma (I mean who plays karma??). I do. And played right, she's viable in everything even in the jungle.
First and foremost, this is to build Karma tanky and offensive!
YOU: Why would I want to do such a thing?
Me: Karma's a support and have you seen supports? Most are soft and squishy. This rectifies that. Also gives her a nice offensive touch by focusing on AP items as opposed to typical support builds.
Here we go.
I'll separate this into 'laning phase' (everyone stays in staic lanes till bout lvl 6), 'roaming phase' (the part of the game where everyone starts moving into other lanes) and 'all mid' (For the first of hopefully many battles).
I do plan on updating this by the time the rework comes, so please DO comment!
And sorry this is so wordy, I'm new to this whole guide thing.

Like everything, this is up to you really.
Keep in mind that Marks are best for physical attack (NOT NECESSARY!!), Seals for armour and health (pretty useful) and glyphs for magic, mana CDR (VERY USEFUL). Might wanna pick up some CDR actually, just in case. Remember it caps at 40% though otherwise unlimited shield bombs.

I prefer a more support oriented build in terms of masteries. You should by now know what each one does and if not feel free to scroll your mouse over it :)
I put less of an emphsis on speed and migic resist but if you like it feel free to swap any you see. I most certainly recommend Block and Indomitable however. Perseverance is also really useful, scales perfect with Karma's passive. You want to stay as close to death as possible without dying since lower health increases AP, one would think having heath regen would counter that but I don't (even though I use this people would disagree). Having a perpetually low heath bar, ie losing and gaining it back quicker, would mean that even though the AP would flux a bit it would always stay high because this, coupled with armour and magic defense, is a great combo.

Bread and butter of every build, this one strives to make a balance between tank and AP. It's important to remember that these items in my standard are meant for Karma for 1v1s or 1v2 against her, I'm not saying you'll live but you'll live long enough for your team, or ADC at least to get there and finish the job. The 'right' items to take is situational, as everything, and should be modified depending on how your enemies are doing.

This is a way to play Karma, it works for me, hopefully it'll work for you. Go full tank, full AP or both (which I strived to create). Remember though, having a large heath bar IS NOT always for the best! since AP boost scales for % of heath missing the more heath =more damage have to be taken for this passive to be viable. Thus if AP, try getting heath regen as opposed to base health.

And remember again, this is really situational... I play like this typically but that's because I know how to, her ulti and when to use it in accordance to certain situations varies. Practice makes perfect.