When you are at the Subway Town and doing the job at the abandoned distillery, you need to go to the room that has lots of very tall stills in it. Look for the ladder that leads down to a part of the floor below the normal level.

Climb down the ladder and follow the route set out around the still. At the end of this route you will find a matress with a valve above it. Interact with the valve and you will open the door to the developer room.

To get to the room however you will need to go back up and look to your right. You will see the door that leads to the developer room openhalf way. Then simply crouch and enter into the room and you will unlock the achievement.