Well... this guide is not very detailed, and it is meant to be a "Summary" of the endings, but anyway, here goes

Normal Final (Classic Style): Simply get all 7 notebooks and grab the 4 exits, and you're done.

Secret Ending (Classic Style): Answer all the questions wrong in the notebooks, and complete the game (Baldi will go faster than in the normal ending).

Normal Ending (Birthday Bash): Grab the 9 notebooks (7 at the bottom and 2 at the top), count the "Baldloons", and wait for the "You Won" screen.

Secret Ending ("Birthday Bash"): To get this ending, in a gift vending machine, you have to get a teleporter, then, you play normal, grab the other two notebooks (so you have all 9), and use the teleporter, then, you go to the principal's office, and there will be Null/Baldi, and well, that's the ending.

Demo Style (Normal Ending): take the 7 notebooks and the 4 exits, and you're done.

Demo Style (Secret Ending): well, this is a long one, you have to get the 7 notebooks, but once you get them, you have to go back to the rooms of the previous notebooks to answer "Bonus Questions", there are 6 of those, once you answer the 6 new questions, you will get a "Portal", that you have to put behind the principal's office (look it up on google, I'm too lazy to put images), if you did it right, you can go to the basement, and get the ending.

NULL Style: in this "Style" you will fight with Null, who is like Baldi, but does not produce sounds, and is much faster, once you get the 7 notebooks and the 4 exits, he will start a "Boss Fight" (with epic music), you defeat him by throwing things, and well, that's how you get the end.

Glitch Style: the same as "NULL Style", but instead of Null, it is a Red Baldloon.

if I have made a mistake in anything I have said, let me know so I can correct it (in fact that is the reason why I put this here and not in the guide section).

in the party style secret ending you only need to blow out the candle before teleporting

and for demo secret put the portal poster as close to the poster and then go into baldi's basement

null and the red baldloon moves more on each interval and will crash your game on death