Earn three stars in every location on the map.
You can earn 3 stars on each of the 5 locations in the episode. The first star is awarded just for completing the level. The second and third stars are awarded for collecting all the sproutlings and Lost Things, respectively. So completing the game and gathering all of the collectibles will net you this achievement. NOTE: You do not need to earn all three stars on the same playthrough of the level, but it is recommended to save time.