As you progress through the game, forest spirits will begin to appear in the Hall of Doors main area. At any point after they have appeared, get one to follow you out through the entrance where the metal detector is - once the little guy walks through the metal detector, this achievement should pop.
Note that this may not be obtainable post game, as the residents of the Hall of Doors move locations after the final boss fight. The easiest time to do this is probably right after the Urn Witch fight, since there's a forest spirit sitting on a bench very near to where you return to the hubworld from the Urn Witch Laboratory door.