Endoparasitic uses the Godot Mod Loader addon. Official documentation for making mods with it can be found here: https://github.com/GodotModding/godot-mod-loader/wiki

Once you've made a mod, to upload it(Windows Only Currently):
Go to steam library, right click on Endoparasitic > Click 'Properties' > Click 'Betas'
Under 'Beta Participation' click the dropdown and select 'modding', then close the popup
Click 'Play' > select 'Launch Steam Workshop Mod Uploader Tool' > Play
Select mod file and preview image and upload