After meeting Forrest Kaysen, you will obtain new clothes for David, Amanda, and Forrest. After that, head to David's bedroom (the room across the hall from the bathroom where you start out) and open his closet in the back right-side of the room.
There you will have the option to change David's clothes. Change something, and press "Close" since you have to back out of the menu in order for it to unlock the achievement.