To get the good ending you just need to beat the game. That's it, simple enough.

If you are struggling to beat the game even on normal, try your best to not piss off Saiko, and submit to her when she wants you to play with her (chase her for keys, rock-paper-scissors, stay close to her when she tells you the code (additionally, if you don't piss her off at all she could tell you the entire correct code)), and let her take you down when she gets on your back or tries to snatch a key from you. She won't go into other rooms, like the power room, and shut off the power but she will occasionally turn off the lights of the room you are in.
She will start to randomly tic and stab you once, but this is when you are about halfway through the code. Her doing this does not count towards her turning yangire if you are trying to get the achievement.
Lastly, do not let her grab the final key from you or it could trigger one of the other endings if the requirements are met.

Almost identical to the good ending except you have to choke at the end.

Saiko needs to be fairly agitated and annoyed to be able to trigger this ending. That means you will have to play the game "normally" (keep her away from you as best you can and only have positive interactions with her if you need to), but when you have the exit key you will have to let her capture you but letting her grab you from behind with the key in hand.
I'm quite sure, but not certain, that she needs to be yangire to trigger it, as I have had her take me down while I had the exit key and she just took it and ran.

This one is somewhat unknown but as simple to get as the bad ending. BUT you must have a little bit of RNG on your side as well as not accidentally triggering the bad ending (I actually discovered how to get this one on accident when I was bored and tried to get the bad ending, as well as unknowingly unlocking the "Be nice" achievement).

For this ending it is recommended to keep Saiko fairly passive, just to keep the risk of getting the bad ending low. This is because it is almost the same route as the bad ending except she needs to take you down somewhat close to the exit door (make sure you had the exit key on you too), but when she runs off with the key there is a chance that she will run directly towards the exit door. Trying to grab the key from her here will immediately trigger the ending.
You could use the baby strategy I mentioned in the good ending tab, but not book it to the exit after getting the key.

Firstly there are the achievements for actually getting the endings:


Escaped! -Good Ending
Headless survivor -Bad Ending
Escaped again -Best Ending

Here are some misc achievements which can be obtained by just playing through these (excluding the ones that you need to die to unlock):


You waited
Wait for Saiko in the starting office

Calm down
As Saiko is turning into yangire, go up to her and calm her

Sleepy!
Have Saiko drug you with sleeping pills

Get Poisoned
Have Saiko drug you with poison

Why did you wake me?
When Saiko sleeps in the office or infirmary, wake her with a sound

You wanna play a game?
Win rock-paper-scissors with yangire Saiko on the last chance

Complete diary
Complete the journal, return it to Saiko and arrange things so you get the best ending

You are lucky
Win RPS with yangire Saiko on the last chance and escape immediately after

Be nice
Get the good or best ending without letting Saiko turn yangire at all

Unscathed
Escape without taking any damage throughout

Saiko Speedster
Escape in 5 minutes or less