After you solve the leaf puzzle, you'll be able to enter the storeroom. In the back of the storeroom there will be a door once you light the lamp in the room. End your game by leaving through the exit for this achievement.
I highly recommend saving before doing this, because this is one of the game endings.
A note from commenter Tachikoma Shell:
You can only enter the door in the storeroom if you solve the leaf puzzle and get rid of the dark barrier inside the storeroom before your torchlight turns red. If it turns red then the door in the storeroom will vanish and you won't be able to get this ending so you have to be pretty quick.