Unmissable achievement.
You get this achievement for simply working your way through the upper part of the sewer.
When you first enter it, turn on your flashlight, go forward and listen to a brief conversation, turn right go down and find a tape and map on a crate on your right.
Go forward to the intersection, and follow the path until you hit the first fork where you can actually make a choice. To your left is an overturned shopping cart, ahead of you another fork. Head forward and look down and to the right. You should see a green arrow pointing through the rough passage on your right. This is the direction you want to go.
Follow it forward to the barrel with the fire. Turn right and follow the path around until you come to another crate with another tape and a note. Keep going forward and you'll come to some stairs. Go down them and forward toward the hatch ahead of you. When you reach it you'll go into a loading screen and the achievement will unlock when the next area is loaded.