Eventually you'll start encountering Butterflies and their distinct Butterfly icon on the map. Simply walk up to a Butterfly Icon and you'll be warmly greeted by a kaleidoscope of Butterflies. Follow the direction they take and try your best to stay as close as possible. After a couple of minutes walking, they'll eventually stop and reveal a hidden chest. Open the chest and the achievement should pop.
A side-quest for Clementine Willardsey will also appear in the "Three Broomsticks" pub once you have completed your first lesson in the Potion Class. During Clementine's quest you'll also need to follow Butterflies to a hidden chest.