This achievement is story-related and cannot be missed.
*** Spoiler - click to reveal ***At the campsite, Clementine will look around for food. After putting a tied-up walker out of his undead misery, she can proceed with opening a can of food (found by searching through the trash bin in the lower right of the area). As Clementine eats, she will have the opportunity to share food with the dog. The dog gets greedy, biting Clementine, who fights back. After you decide whether you want to kill the dog or let him suffer (Sarah McLachlin would be so upset with that choice) Clementine will nearly pass out in the woods. Upon trying to flee oncoming zombies, she will be saved by some new passersby. The achievement "On the Path" unlocks at this point.