BepInEx

You can download BepInEx below and the software is needed as a mod loader for many, many video games (any on Unity 3 or newer).

Supported Operating Systems

Windows 7, 8, 8.1, and 10 (both x86 and x64 are supported)
Linux distributions with GCC 10 or later, preferably a GNU/Linux distribution (x86_64 and x86 architectures are supported)
macOS 10.13 High Sierra or later
Other OSes that can be used via hardpatching
Any OS with Wine support (Linux, macOS, FreeBSD, …)
Any OS with Mono support (Windows, Linux, macOS)

BepInEx Installation (Windows):

Download the correct version of BepInEx.
Download BepInEx from one of the available sources.
Choose the version depending on your OS:
x86 for games with 32-bit executables
x64 for games with 64-bit executables
Extract the contents to the root directory of the game.
Once you have downloaded the correct version of the game, extract the contents of the archive to the game folder. (The root folder of the game is where the game executable is located.)
Perform the first run to generate the configuration files.
Simply run the game executable. This should generate a BepInEx configuration file in the BepInEx/config folder and an initial log file BepInEx/LogOutput.txt.
Configure BepInEx to suit your needs.
Open BepInEx/config/BepInEx.cfg in a text editor of your choice. All options are documented directly in the configuration file.