From lstwomods.com

Go to the Downloads page and download with dependencies (or without if you plan on installing it alongside other mods and know what you’re doing).

Then extract the .zip file after it finished downloading and copy the contents inside.

Open the game’s folder through steam by right clicking the game in the list and going to Manage > Browse Local Files. There paste the contents of the .zip so the game’s folder should look like this:

<Game Folder>
├── <Game>.exe
├── <Game>_Data/
├── ...
├── winhttp.dll
└── BepInEx/
    ├── core/
    │   └── <BepInEx files>
    └── plugins/
        └── lstwoMODS/
            ├── <lstwoMODS files>
            └── Overlay/
                └── <lstwoMODS overlay files>
     

Manual Installation

Download lstwoMODS Core from GitHub (don’t download source code) and extract the downloaded .zip. Download BepInEx 5 from GitHub (download the one with win_x64).

Copy the contents inside and do the same procedure from above for both lstwoMODS and BepInEx.