Story related, cannot be missed.
This description will pretty much spoil the beginning of the game for you so BE WARNED!
*** Spoiler - click to reveal ***After beginning the game you'll be greeted with recap of Season One, followed by a short scene with Omid and Christa. After two dialogue options you will enter a restroom by yourself. Check the stalls (nothing to find here) then clean yourself up using the water from your bottle. After Clem drops the water you can search the stalls for it. At this point a teen girl will come in and start going through Clem's things as she hides in the stall. After a (naturally) heartbreaking scene here you'll be dropped back into The Walking Dead.
The next scene takes place 16 months later. After some dialogue with Christa she will wander off in search of wood. Look through your bag for the lighter and light the fire. You are now free to roam the (very small) campsite. Pick up the log resting near the fire pit to use for kindling if you don't want to burn the drawing or photo. Once you add some fuel to the fire, this will prompt a scene in which Christa is being questioned by some strange men. You can choose to either leave stealthily or distract them, I chose Distract and was then involved in an intense scene where you will have to escape from a stranger pursuing you. While the UI is graphically different from Season One, the controls will be very familiar to anyone who has played these games before.
Once you escape from the stranger and some Walkers you will fall in the River and wash away. At this point you'll hit a loading screen and the achievement will unlock.