You can install Boomer mod with MelonLoader and KarlsonLoader. To install with MelonLoader,

1. Get a fresh install from itch.io of Karlson (https://danidev.itch.io/karlson)
2. Download MelonLoader 0.3.1 (https://cdn.discordapp.com/attachments/837364698949091328/837370417487413338/0.3.1.zip)
3. Unzip the contents of the archive in your new install of Karlson.
4. Get the MelonLoader (BoomerMod_ML.dll) version of the mod (https://github.com/karlsonmodding/BoomerMod/releases)
5. Place it in the Mods folder.

To install with KarlsonLoader,

1. Get a fresh install from itch.io of Karlson (https://danidev.itch.io/karlson)
2. Download KarlsonLoader (https://api.xiloe.fr/karlsonloader/KarlsonLoader.exe)
3. Place KarlsonLoader in an EMPTY directory and run it.
4. After launching KarlsonLoader and selecting your Karlson install, close the game
5. Download the UML (BoomerMod_UML.dll) version of the mod (https://github.com/karlsonmodding/BoomerMod/releases)
6. Place the mod in the UML/mods folder of KarlsonLoader
7. Download the dependencies (KL_deps.zip) of the mod (https://github.com/karlsonmodding/BoomerMod/releases/)
8. Unzip the contents of the archive in the UML/deps folder of KarlsonLoader

Note: Both mods have the exact same behavior, and both are allowed for runs.

what mods folder?

no idea what you mean by mods folder