This achievement is story based so it is unmissable however I will post the quickest way to get through the chapter if you don't want to spend time exploring for back story.
Meet Lily, Larry, Doug, Carley and Glenn.
There will be a fight between some of your group and some of the new people you meet over whether or not Duck is bit. Choose whatever you feel is right here as it won't affect the guide or this achievement. During the fight Clem gets attacked by a walker, follow the onscreen instructions to run and save her.
Pick up the radio that Carley is working on and turn it over to reveal the battery door. Pop the battery door open to discover the reason the radio isn't working... The batteries are dead (duh Carley)
Battery 1 is on the shelf behind the greeting cards
Battery 2 is on the floor around the corner in the middle of the room.
After that go into the employee only section and move the wooden pallet and the desk blocking the door. When Clem cuts her finger find the first aid kit on the nearby desk and open it to reveal a bandaid. Go back to Clem and bandage it then examine a picture on the ground to get a cutscene with Carley and an item you will need later (Picture proving trapped walker worked at Pharmacy) Open the drawer under Clem to get a TV remote (It will come in handy later)
Glenn will call you on the walkie talkie when you exit the office, find Carley and tell her you are ready to go.
As soon as you gain control of Lee you will behinda vehicle in a motel parking lot. Lean left and pick up the pillow that's on the ground. Lean back to the right and run to the nearby truck and examine it to reveal a screwdriver. You cannot break the glass to get to it quietly but you can use the pillow to smother the nearby zombie leaning against the car. Just highlight the zombie and follow the instructions. Open the car door that the zombie was leaning on to get a spark plug. While you're there move the gear shift to smash a zombie against a wall. Return to the screwdriver and use the spark plug to bust out the window (I don't know how that is any quieter than using your fist but whatever)
Return to the sign at the start of the level and highlight the nearby walker and use the screwdriver option, follow the onscreen instructions to kill the remainder of the zombies. After this you will go up to save a trapped girl, choose to help her kill herself or not, doesn't matter.
Achievement will pop after she dies.