And then once youve unlocked the thief ability as shown in King fiShiZ's guide, you then need to hire a group of theives, these are dotted around the city, or near a thief designated faction building or near the thief HQ marked by the mask icon on the map.
Once you have your thieves you then need to set about killing guards, any will do as the thieves will follow you everywhere. Just kill guards then they will come and loot the guard for you and the spoils of war will pop up on your screen, just like you looted them yourself.
DO NOT get them to lure guards as they will probably dissapear
Hope this helps as it took me a while to work out what it meant.