This game requires 2 playthroughs for the 2 ending achievements.
Each playthrough must be done in one sitting. While you can go back to the menu safely, there is no save feature if you exit the game.
There is plenty of time and no need to rush.
Nightfall is over real time (about 45 minutes per playthrough).

The first playthrough can be completely blind. Anything missed can be picked up on the second playthrough.

While I have noted the "Treasure Hunter" quest on the map below, his quest is OPTIONAL and may prevent you from getting "Sad end".
If you got "Happy end" with the artifact on your first playthrough, I would recommend not picking it up on your second playthrough.

Your final dialogue choices decide which ending you get. If you only need the other ending achievement, you can just get to the forest and wait ~40 minutes for night.

Controls:
'F' to interact/talk to people.
'M' to bring up the map.
'J' to bring up the quest log.
Hold 'Shift' to run.

In your apartment:
Check the computer.
Take the map.
Talk to the girl on the bed (top option).
Leave through the door.

You'll then be transported to the forest. Following the path, you'll come across an arguing couple.

(Optional) Talk to the couple and receive a quest.

To the right, there's a small pond and campfire.
Talk to the men. The Dancing Man will give you a quest. (required for "Today's Weather")
Pick up the water bottle.

Go to the Ancient Ruins in the northeast corner.
Fill the bottle and talk to the Praying Woman.
Finding the 4 tablets for this quest will give "Who are the Ancients". (See map below for tablet locations)

Going counter clockwise around the map:

Talk to the hurt woman on the path. She asks you to bring her bag from nearby ruins. Go get the bag and bring it back to her.
After this quest is complete, she asks: "Are you single?"
Answer "Yes, I am" and she'll give you her number.

Cheater
Are you a cheater ?


Read the tablet in the circle ruins in the northwest corner.

(Optional) Talk to the fisherman and go swimming for his quest. Pick up the purse for the couple.

Head to the southwest corner and read the tablet in the pillar ruins.

Head east and find the deer.

Find an animal
Where are the animals?


Head further east and find another tablet in a small ruin and "The funny man" on the nearby hill.

The funny man
Talk to the funny man


Go north back to the camp by the pond. (Turn in the purse to the couple along the way if you wish.)
Talk to the Dancing Man again and receive a key.
Go into the weather station and read the report on the right table.

Today's weather
The weather seems unstable


Head west to find the last tablet on top of a hill and wait for night.

Who are the Ancients
Find all the parts of the Ancient story


Note: This popped for me after opening the quest log (J) and waiting a few seconds after finding the last tablet.

Endings:
After night falls, everyone will leave and you will no longer be able to quest.
At "What the hell is that glow in the sky? Am I dreaming?", you can look up to watch a UFO land.
Go to the UFO area and wait for the Ancients to talk to you.

When you're asked "Do you have the artifact?": (The Artifact is from the Treasure Hunter quest, NOT the tablets for the Praying Woman.)

Tell the truth and go with the Ancients for:

Happy end
Discover an alternative ending to the game


Or lie about having the artifact and don't go with the Ancients for:

Sad end
Discover an alternative ending to the game

Cheater
Are you a cheater ?


Talk to the Hurt Woman on the path and bring her the bag from nearby ruins.
After this quest is complete, she asks: "Are you single?"
Answer "Yes, I am" and she'll give you her number.

Find an animal
Where are the animals?


The animal is in the middle of the south half of the map. See map below for exact location.

The funny man
Talk to the funny man


The funny man is located in the southeast section of the map and can be spotted from Tablet 3. See map below for exact location.


Today's weather
The weather seems unstable


First, talk to the Dancing Man by the pond. He requires you to take the water bottle next to him and fill it at the Ancient Ruins in the northeast corner. Bring it back and he will give you access to the weather station.

Enter the weather station and read the report.

Who are the Ancients
Find all the parts of the Ancient story


Talk to the Praying Woman at the Ancient Ruins (northeast corner) and find the 4 tablets (marked on the map below).
Note: This popped for me after opening the quest log (J) and waiting a few seconds after finding the last tablet.

Happy end
Discover an alternative ending to the game


Tell the truth at the end and go with the Ancients.

Sad end
Discover an alternative ending to the game


Lie at the end and don't go with the Ancients.