In order to run Enter the Gungeon mods, you need to go through the process of installing Mod the Gungeon. Instructions for downloading Mod the Gungeon can be found on the official site.

1. After downloading both the mod and Mod the Gungeon, navigate to your Enter The Gungeon directory, commonly located at "C:\Program Files (x86)\Steam\steamapps\common\Enter the Gungeon", and drop the mod .zip file into your mods folder
2. Delete mods.txt in the mods folder.
3. Boot up the game and press f1, you should see the mod in the "enabled mods" list. Additionally, if you press ~ an initialisation message would most likely be in the console.

It is also recommend that you download Gungeon Go Brr for perfomance, instructions for which can be found on the modworkshop page for Gungeon Go Brr.