Pick up the Contract: Imp off the notice board in Hierarch Square in Novigrad. This will lead to the An Elusive Thief quest.
According to some posts on the internet this achievement may not pop. I thought that was the case for me, but...
*** Spoiler - click to reveal ***I took the first option of letting him live at the end and the achievement didn't pop. Later I went back and killed him and the achievement popped.