Here are three important facts about Karma that this build is based on:

• Karma's skills scale greatly on AP (60% Heavenly Wave + 1% heal per 50 AP / 70% Spirit Bond / 80% Soul Shield).
• Her cooldown are relatively low and it is worth getting CDR items to improve her spamming abilities as well as the ability of using her Ultimate Mantra. Many time these low cooldown save her and her teammates' lives.
• Because of these spamming abilities the needs to have enough Mana Regeneration.

Ealry Doran's Ring and Fiendish Codex provide her a considerable amount of AP early on. They also add the important Mana Regeneration and already the first 10% CDR.
Athene's Unholy Grail give improves this a lot and adds Magic Resistance plus increased Mana Reg when out of Mana.
The following items built bring more AP, Magic Penetration and CRD as well as survivability.

Make sure to have all skills in you inventory as early as possible to be flexible and supportive early on. Them max Soul Shield and Heavenly Wave that give you more damage outpunt and support efficiency.