You'll get this achievement after you have checked out the barn. You need to check out the doors at the back of the barn. Kenny will come and talk to you, then Andy. Go outside to the work bench and take the multi tool, approach the generator and remove the panel. You then need to turn it off and remove the belt. Andy will come outside to check on it. Go back into the barn.
The dinner bell will ring, and you can check out the back room by removing the screws. Once in the back room you will be approached by Andy and taken into the house. When you go to wash your hands, sneak upstairs and open the closet. Use the power cord to turn on a light. Go into the nearby bedroom and move the bookcase to reveal the source of the light.
*** Spoiler - click to reveal ***You find Mark, who has had his legs cut off. Everyone downstairs is about to eat them! Run downstairs and shout "CLEMENTINE. NO..." to stop her eating. After the rest of the choices and conversation you will get the achievement.