You need to activate all the memories(voice acted bits of story) in the game to unlock this achievement, aside from the one that's only in the Apathy ending. (See the "Apathy" achievement.)
The memories are as follows(not necessarily listed in the order of the game):
a. Going to the pond across the road. - See the "Bliss" achievement.
b. Entering the house.
c. Entering your room inside the house.
d. Entering the wooden shed with the stencil on the left side of the house. - See the "Sown by the Mountain" achievement.
e. Going close to the wooden fence on the left side of the house, where the dog food is. - See the "Dakota" achievement.
f. Finding your fort made up of corrugated steel plates, on the left behind the house.
g. Walking on the path in the forest towards the field beyond the house.
h. Entering the cemetary in the field beyond the house.
i. Having found all the previous memories, another one will automatically play, prompting you to find a key in the house.
j. Picking up father's flask on the ground a ways behind the house. (Secret memory.) - See the "Good Intentions" achievement.
k. Getting the key from the drawer in father's bedroom, then unlocking the door to the shed to the right of the house, and riding the bicycle.