NieR Replicant ver.1.22474487139: All Endings Guide

This is the easiest. Simply beat the game. Credits will roll and you will get the A path acheivement. From this moment onward, every time you start a new ending path, it will start you in Act 2 of the game.

Simple as well. Simply beat the game after loading the save from beating the game once. This file will have an A logo on the save screen for you to differentiate with any other save.
This new path provides you with a lot of context that was missing from cutscenes. Play through the game once more until the credits for an added cutscene.

This is where it gets a bit complicated. Reload your save file after clearing ending B (it will have an A and a B on the save icon). This time, as the game states, you will need to collect all 33 weapons in order to unlock this new ending. Make sure you collect every single weapon from quests and from dungeons as well as given ones. Please keep in mind this also means weapons from Act 1 of the story. If you did not obtain these, there are many that can still be bought in Act 2, but not all. Take the time to get everything in Act 1 before continuing if you are reading this in advance.

This path provides a bit more added context and new scenes but only to some cutscenes.
Create two saves before the final boss. Choose to Kill when the option appears. This will end the game with ending C.

Reload your second save from before the final boss. Go through the motions and when presented with the same choice from Ending C, choose to spare them. This will provide you with a new ending and Delete all your saves. Don’t worry..

Ending E
When your save file is deleted, your login screen will show up with a different icon, this means you can now “activate” the new ending. Create a new game file with a NEW NAME (not the same as whatever you used in your playthroughs) for your character. Play through the game until you get to the point where you fight the giant monster in the Aery for the second time, where Kaine would normally join your party. This is where things will play out differently and you will get a new ending.