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 arrive at Hershel's farm and gain control of Lee speak to Hershel on the front porch, after that small cutscene/conversation you will speak with Clem in the barn. In the morning you will meet Kenny and Duck. After you meet them find Kenny in front of his truck and ask him of his boat and his plans, then talk to Katjaa and Clem near the front porch right next to Kenny. After talking to them go around the house and find Shawn working on the fence, ask if he needs a hand and cut some wood for him. Afterwards go back to the barn and speak with Hershel which will trigger an attack on the farm.
During the attack you can choose to save Shawn or Duck. SHAWN DIES NO MATTER WHO YOU CHOOSE.
After your choice Kenny will offer you a ride to Macon, accept it and the chapter will be over.