All of these achievements are unlocked upon completing a run, granted you follow the specific conditions mentioned for each one.

• Ending #1 - "Stranger Danger": Do not let the Stranger enter. ("The Stranger" is the shadow person that appears half-way through the game and watches the player for a short while before driving away).
  
• Ending #2 - "Stranger Things": Let the Stranger enter.
  
• Ending #3 - "The Savior": Do not let anyone enter.
  
• Ending #4 - "Rite of Passage": Unlock endings 1, 2, and 3, then simply start a new game.
  
• Ending #5 - "Missed Call": Don't answer the phone call from Natasha's ghost.
  
• Ending #6 - "Curiosity Killed The Guard": Walk in front of a moving car as it approaches the gate behind the security booth. The game will end when the car hits you.
  
• Secret Ending: As soon as you let the Stranger in, you must open the exit gate (if it's not already open), attempt to walk inside, and select "Yes I'm sure!". A special cutscene will play and the game will end.

• Employee of the Month: Only let Nova Nexus employees in. Simply check the register for each car that drives up, and refuse entry to any cars whose license plate numbers do not appear in the register.
  
• Pro Guard: Only let Nova Nexus employees in, this time without checking the register. In most cases, cars with license plate numbers following the format "Letter Letter Number - Number Number Letter" can safely be identified as Nova Nexus employees. Some cars will have special names that do not follow this format, however.
  
• Booth Bae: During the second section of Ending 4 (the part in which you are exploring a small parking lot collecting coffin nails from the restless souls of the deceased Nova Nexus employees), check the back of the mausoleum reached after opening the gate; you will find a framed picture of the iconic security booth with hearts stamped on top of it. Interact with this picture to collect it and earn this achievement.

• Watched "Mental Notes": Complete the bonus tape "Mental Notes", unlocked by getting Ending #1 in the main game.
  
• Watched "Lockdown": Complete the bonus tape "Lockdown", unlocked by getting Ending #2 in the main game.
  
• Watched "Stations": Complete the bonus tape "Stations", unlocked by getting Ending #3 in the main game.
  
• Car Collector: Collect all 6 toy cars found within the bonus tapes. There are 2 cars for each tape. This task may prove somewhat difficult as the toy cars are, true to their name, very small. Zooming in (activated by holding the right mouse button) and using your flashlight (toggled on and off using F) can help make some of the cars more visible. When you find one, simply interact with it to pick it up.

For those who struggle to find all of the cars for the achievement "Car Collector", here are the actual locations (with pictures) of each one.
As you make it across space towards the door at the end, there is a clutter of office furniture and such to your right. Walk towards it and just slightly above you (might take some position adjustment to see it) should be the car. (Author's note: My apologies for the car not actually being visible in this picture, I had just collected before I realized I was gonna make an achievement guide. I made sure to try to point the camera towards its general location).
After walking through the door you will enter an office with many, many screens. Walk towards the desk to the left-forwards of you and the car will be on the back-left corner of it.
Once you reach the 4th office containing 6 blue screens on the wall, position yourself to be directly in front of the screens then turn around. The car will be behind the middle computer of the front row of desks.
After the lockdown begins, make your way to the next office, which is just past the closing security door with the ceiling turret gun. Enter the office, then check the folder drawers behind where the door is when you open it. The car will be on top of the middle drawer stack.
When you connect to the Kraken station, take a right, then walk until you see the empty display section which will be, again, to your right. The car will be on a box beside the desk with the computer on it.
When you connect to the Medusa station, walk through the door at the left end of the closet, then head straight to the second-right window with a fallen desk next to it and a blood-stained mountain of snow coming through it. The car will be right where the snow goes through where the glass used to be. If you can't see it initially, zoom in or walk through the snow until you get stopped by the wall, which will place you directly in front of the car.

Hope this guide helped you get all achievements for this wonderfully crafted horror game! Comment down below if there's anything that you think should be clarified, fixed, or otherwise changed.
Special thanks to "Vecreagler" for the Pro Guard guide and "ᐯㄖ丨ㄒ卄" for the Endings guide, as well as some posts on the discussion boards that helped me find a couple of things I wasn't able to on my own. Edit: I also recommend you check out "Scrimbo"'s guide containing all of the license plate numbers that can be admitted into Nova Nexus if you're still stuck on the "Pro Guard" achievement.