To unlock this achievement you have to find the Jones Mask at the end of Chapter 5, Full House. First, kill everyone in the building so that the "Go to car" message appears. Instead of going to the car, pick up the crowbar lying near the entryway and run around to the back of the building.
There will be a manhole which can be opened with the crowbar (simply walk towards it while holding the crowbar). Inside, the mask will be lying next to a mortally wounded man. Pick it up and finish the level, then revisit the sewer again in another playthrough while wearing the Jones Mask to get the achievement.