Text Editor

Notepad++ is the Recommended Text Editor to Use

.entities files are compressed and must be decompressed in order to edit Entities.

Decompressing Tools:

EntitySlayer by FlavorfulGecko5 - Download
File -> Export to text (make sure the file extension is still .entities)

SAMUEL Asset Extraction Tool by SamPT - Download
Currently exports textures, .decls, and .entities files.

idFileDeCompressor by PowerBall253 - Download

Even though the .entities file is decompressed, make sure it still retains the .entities file extension.

When editing a decompressed .entities file while you are in game, you will need to create a folder called "overrides" in the DOOM Eternal installation directory. Structure it like this:
overrides/maps/game/<campaign>/<level_name>/<level_name>.entities
Example:
overrides/maps/game/sp/e1m1_intro/e1m1_intro.entities
> This is the level file path for Hell on Earth.

You can place as many level files you like in the overrides folder

With M347h00k installed, you can enter your level and edit the decompressed entities file while you are in-game.
Typing in the console command, mh_force_reload will reload the level with the recent changes you made in the text file.