Thief looting is a reward you get for completing Set 2 of the Thieves Challenges.
Perform 40 leaps of faith
Climb 1km in distance
Steal 250 currency with Counter Steal
You can use any leap of faith over and over again. I would just suggest finding one that is easy to climb to and using it. This might come over the course of gameplay as well. For climbing 1km, this will come easily while playing (considering you need 40 leaps of faith as well). To steal money, hold and press when a guard attacks you. You can check your progress of these challenges by pressing and selecting challenges.
Now that you have your thief looting skill, find some thieves and hire them. Now go kill guards and they will loot all of the bodies for you.