At the end of the episode, after all of the intense fighting and backtracking back to the school grounds, you will be in control of a character as they goes about some chores for the community. There will be some conversation, and eventually you'll head inside toward the character's bedroom. This is your opportunity to place the episode collectibles around the room.
At the end of the room opposite the door, there will be a desk by the window and you'll have the option to place Clementine's hat. Save this for last as doing so will end the act, episode/DLC, the game, and the series, and the achievement will pop.