The objective of this guide is to save you the time of making a roadmap of all the items you need to collect, where you need to use each of them, and where to optimally save in order to see all the NG+ endings (Dog, UFO, Bliss, Stillness & Rebirth) in a single playthrough. Just remember that you first need to achieve the In Water ending before you can get Stillness

I will be mentioning each item, their location, and which ending they correspond to, in order of appearance as you play through the game.

• Crimson Ceremony
  Location: Graveyard (After meeting Angela)
  Ending: Rebirth

• Key of Sorrow
  Location: Green car (After getting the chainsaw, break one of the windows to get it)
  Ending: Stillness

• Broken Key Part
  Location: Pet store (North of Big Jays once you get to the city)
  Ending: Dog

• Blue Gem
  Location: Jewelery store (Also north of Big Jays, a little further than the pet store)
  Ending: UFO

• Use Blue Gem
  Location: Roof of Saul Apartments (You will hear a weird sound, when you investigate the gem it must shine like in the photo)
  Ending: UFO

• Use Blue Gem
  Location: Rosewater Park (Before you meet Maria, you'll hear the sounds again)
  Ending: UFO

• Rusted Key
  Location: Bowlarama (The code is 1887)
  Ending: Bliss

• White Chrism
  Location: Baldwin Mansion (Right after you pass through the motel)
  Ending: Rebirth

• Broken Key Part
  Location: Katz st. (On the right side of the road, there's a small entrance. Look for the red car)
  Ending: Dog

• White Claudia
  Location: Gazebo in Brookhaven Hospital (You can pick it up as you're going to the pool, you'll get the chest and must combine it with the rusted key)
  Ending: Bliss

• Obsidian Goblet
  Location: Historical Society (Replacing one of the paintings, adjacent to the wall with a missing painting)
  Ending: Rebirth

• Use Blue Gem
  Location: Lakeview Hotel Pier (In the stairs as soon as you get off the boat)
  Ending: UFO

PROCEED WITH THE GAME UNTIL YOU GET THE KEY TO THE 3F PADLOCK

(Make sure not to pick up anything from the Lost & Found room yet)

• SAVE YOUR GAME IN A DIFFERENT SLOT
  Location: Lakeview Reception

• Open the Door of the Observation Room with the Dog Key to Watch the Ending
  Location: Lakeview 3F
  Ending: Dog

LOAD YOUR SAVE

• Use Blue Gem to Watch the Ending
  Location: Room 312
  Ending: UFO

LOAD YOUR SAVE

• Drink White Claudia & Play the Tape to Watch the Ending
  Location: Room 312
  Ending: Bliss

LOAD YOUR SAVE

• Toluca Postcard
  Location: Otherworld Manager Office (You'll get here after watching Angela's final scene, the code is Right 3, Left 1, Right 4. Once you obtain the Postcard examine it and finish the game to watch the ending)
  Ending: Stillness

LOAD YOUR SAVE

• Book of Lost Memories
  Location: Lakeview's Lost & Found (Accessible through a breakable wall in the bar, you must obtain this item and finish the game to watch the ending)
  Ending: Rebirth

https://youtu.be/OEoLRuSMAUU

This is a full playthrough, please pay no mind to the silly mods lol

Hope this has been useful, I haven't seen anyone do it yet so I decided to give it a try and make my first guide. I'm not a native english speaker so please let me know if there's any error in what's written.
Thanks for your time!