Mod the Gungeon Tutorial: Introduction
Mod the Gungeon is based on the concept of backends and mods. Backends access the game's code and provide an API for mods to use - sometimes including workarounds or simplifying the usage. Mods, on the other hand, use the APIs (Application Programming Interfaces) provided by backends to add actual features to the game.
The very base backend of Mod the Gungeon is ETGMod. It not only implements the standard API but is also responsible for managing both mods and backends; backends implement APIs using the ETGMod API.
It's very important to understand this separation, as mods can be loaded during runtime while backends have to be injected into the game using the special installer (ETGMod.Installer).
The installer by default installs ETGMod along with selected backends from the repository and has an option for installing backends outside the repository (unsafe, unless you trust the source).
You can now move on to Installing ETGMod, the base backend.