I know its alittle long but's step by step what needs to be done.
I will skip conversation pointers, as those choices are entirely up to you.
After some conversations and cutscenes, you'll find yourself going for a battery with Molly.
Open the door behind you.
Follow the hallway, after Molly. Open the exit to the alleyway.
There's a small white shed with a sign 'looters will be shot on sight', enter it.
Look at the hole in the ceiling.
Climb the shelves.
Try to open the garage door.
Look at the fence at the back of the alley, and walk back. A walker should fall from the roof.
Open the garage door with the quick-time event.
Open the hood of the truck. Notice there's no battery. (And that the elevated car has a blinking light, indicating it will have one).
Try to operate the lift panel (to the left of the car).
Try to grab the hydraulic hose. You'll notice you need something to cut it with.
Ask Molly for the 'hook'.
Use the climbing pick on the hydraulic hose.
Before you take the battery, you'll need to Unscrew and Detatch both the negative and positive terminal (the two wires near the left and right bottom of the battery). After detaching both wires, you can pick up the battery.
Follow Molly up the truck; there's a ladder behind the driver's door.
Shoot the skylight.
Grab Molly's hand.
As soon as you start running towards Molly, move your cursor over her hand and grab it. This needs to be done pretty quick, so press A as soon as you get the hand indicator.
Once inside the school, walk towards the camera.
Kenny needs help with the doors: First there's a quick-time event, then you'll have to shoot the walker that's got his head in the door, then you'll have to pick up the hatchet and perform another quick-time event.
Once you're in the main room, open the door to the hall again. You can have some optional conversations first.
Walk down the hallway, but now (after the big door with walkers) move down the left hallway instead of the right.
Shoot all walkers. Go for the heads and be quick, you don't have much time.
Once in the nursery, look at the medical file on the desk to the left. You'll take a small tape from it.
Use the tape on the camcorder; it's to the right aimed at the bed/cabinet.
Use the hallway door.
Get to the exit near the auto repair - that's first towards the camera, then all the way to the right.
Open the exit door to the alley.
Once again, get into the small white shed.
Before you can climb the shelves, you're attacked by a walker. Dispose of him with a quick-time event.
Climb the shelves.
Crush the walker's head under your boot.
Search the doctor's corpse. You'll get another tape, and a code that's not the cabinet's code.
Walk back towards the nursery.
This part is optional: Halfway towards the nursery, there's a 'bloody locker' (near the hole in the ceiling). It's the doctor's, and if you want you can now open it using the combination you found on his body. You'll get another tape, one which gives you some extra background for Molly.
Enter the nurse's office. You'll watch the tape revealing the combination.
If you want to (and took it), you can use the third tape on the camcorder.
Exit to the hallway.
Walk back to the main room - that's all the way towards the camera. You'll enter a cutscene with Molly before you're there. If you've taken the third tape and watched it (as discussed above), you'll enter a conversation with Molly in which you can get some extra backstory from her.
You won't be able to shoot the walker attacking Molly... But Clementine will!
If you had decided to leave Clementine at the manor, nobody else will shoot the walker - Molly will run away chased by walkers, and that's the last you see of her in the episode. Thanks to Zekinder for explaining what happens here without Clementine. Apparently, if you didn't bring Clementine, you're also able to shoot the walker yourself. Thanks to sunbugsie for pointing this out.
After the conversations and cutscenes, walk backwards up the stairs while giving extra airholes in as much walker heads as you can.
You'll get stuck in the stairs. You'll have to alternate shooting walkers down the stairs and grabbing your leg to free yourself. If you're quick with the two walkers already on the stairs, you should have enough time to free your leg (it took me four tugs).
Move up the stairs. Make sure to use the hatchet on the walkers before you reach any of them. The hatchet will stick into the last walker in front of you, and you'll have to shoot him.
Use a headshot on the walker grabbing Ben.
Another big choice in this episode: What will you do with Ben?
After this choice, you'll receive the achievement. Even if you save Ben, you won't have to take care of extra walkers. The next chapter is pure conversation, so I won't be putting up a guide for it.