Very easy to do. On the Joshua Tree level, swallow a few of the pink pebbles until you're big enough to swallow the campfire. With the campfire in the hole, move towards the crow with popcorn in its beak. The crow will pick up and throw embers when you're near it, and catching one of these will stoke the flames of the campfire in your hole. Once on fire, simply move your hole underneath Pepper's trailer and the achievement is yours.
Do not worry if you missed this during your initial playthrough as level select is available once you complete the game.