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.
When you gain control of Lee run to the front door to help block it off, you will have to follow the onscreen buttons to keep shutting the door. PAY ATTENTION HERE so you hit the correct buttons. Clem will go find your dad's cane to seal the door. A cutscene will ensue and you must choose to save Doug or Carley (Doesn't matter which for the achievement). After you choose follow the onscreen guide to save Clem from another zombie attack then run like hell towards the back door. You will fall.... (again...) and you must grab Kenny's hand to finish.
Achievement will pop after making the choice on who to save.