Some minor spoilers in this section related to achievements. These are blocked as spoilers.

Stargazing:
Complete the Friary Rd chapter. Aka the prologue.


Unknown Geometry: Found some unknown geometry.
Automatic, in the story. You'll see some weird geometry thing on the bed. Poke it >=]


Have a BBQ: Had the last barbecue at no. 29.
Automatic as the story progresses. You get this after the BBQ.


Play a Game: Played Among the Leaves.
Play the game with your friends. Automatic as the story progresses.


Try to Sleep: Had a long conversation in bed.
Automatically when Ao and Bo are talking in bed after the BBQ.


Leave the House: Finished No Longer Home
You left the home, success! But seriously, just finish the game ;)


Answer a Call: Answered the ringing phone.
In Among the Leaves. When walking around the forest you'll hear a phone ringing. Ignore the prompt to go to the house and follow the sound of the phone. Pick the prompt to try and use the pay phone in the forest. EDIT: You need to choose either "Answer a Call" or "Trip to the Shed", you have to playthrough twice for both achievements for 100%. Thanks to LiedHart for the clarification suggestion!


Trip to the Shed: Visited the shed.
In Among the Leaves. Go straight at the beginning and you'll find the shed.EDIT: You need to choose either "Answer a Call" or "Trip to the Shed", you have to playthrough twice for both achievements for 100%. Thanks to LiedHart for the clarification suggestion!


Meet Esther: Found Esther in the plant room.
In Among the Leaves. Go towards the cave, then down the elevator. You'll meet Esther there.


Meet Aya: Met Aya at the shrine.
In Among the Leaves. Go towards the street instead of the cave. You'll meet Aya at a shine there.


Meet Lu: Met your new flatmate Lu.
Right before the end of the game when you are about to leave the home. This is the only time the door at the end of the hallway is open and you will get a prompt to leave the home or stay a while longer. At that point, go into the room right before the exit, the flatmate's room. Next to the studio. There you'll meet Lu.


Meet Gi: Visited Gi's abode
When you are controlling Ao at the point when Bo has gone to bed because they aren't feeling well. You can finally go into that odd room under the neighbor's stairs in the hallway. It's across from the bathroom, studio and flatmates room. Move the perspective around to see it. Inside this room you'll meet Gi. Yay baseball?


Let me know if you see any mistakes. I made this quick right after playing the game when I saw there weren't any guides yet. I may have missed something.