This page explain how to install mods on Cult of the Lamb using BepInEx
Go to the download page of BepInEx and download the last version for your computer : https://github.com/BepInEx/BepInEx/releases/tag/v5.4.21
On steam :
Your folder should look like this :
It will initialize BepInEx for the first time, you can then close the game again
Download the mods you are interested in and move them into the plugins folder, which can be found from the root of your game folder from BepInEx → plugins :