When you first wake up in the game, next to the camp fire, look at it for a "Roast Marshmallow" prompt. Once you get your mallow on a stick, extend stick into the fire, aiming with your right analogue stick, but don't cook it for too long, or it will melt away. Instead, push X to extinguish the marshmallow, then eat. Push X to replace the marshmallow, cook it again, and eat it. Repeat this for a total of 10 times for the achievement to unlock.