I will skip conversation pointers, as those choices are entirely up to you.
You're outside, looking for Clem.
Clem is in the shed, but you can't get to her until she starts banging the door. Seems to me this only starts after you've looked around enough. I looked at the doors of the shed, at the dog's grave, into the doghouse, into the shrubs next to the house; and then when I looked at the fence Clem started knocking on the door. This is not a hard criterium though, it's possible Clem starts knocking before you search all places.
Once there's a sound form the shed, you can open the shed door.
After some conversations; you'll get your next big choice of the episode - Are you taking Clementine with you to Crawford, or do you leave her alone in the house with Omid?
After all conversations, you're in the sewers underneath Crawford.
Open the manhole, and get out into the street.
Sneak up to the guard.
Chop the walker.
You'll get into the school, and the achievement will unlock.