Download modding framework BepInEx
Extract and paste its content into the root of the game. Start the game once to generate necessary files.
For debuging purposes edit BepInEx/config

[Logging.Console]

## Enables showing a console for log output.
# Setting type: Boolean
# Default value: false
Enabled = true

Download ILSpy for decompiling the game.
Optionaly install UnityExplorer in order to live debug and change variables in runtime.

Adjust in MyFirstMod.csproj lines to point to your game installation.

    <ItemGroup>
      <Reference Include="Assembly-CSharp">
        <HintPath>..\..\..\..\..\Program Files (x86)\Steam\steamapps\common\Lethal Company\Lethal Company_Data\Managed\Assembly-CSharp.dll</HintPath>
          <Publicize>"true"</Publicize>
      </Reference>
      <Reference Include="UnityEngine">
        <HintPath>..\..\..\..\..\Program Files (x86)\Steam\steamapps\common\Lethal Company\Lethal Company_Data\Managed\UnityEngine.dll</HintPath>
          <Publicize>"true"</Publicize>
      </Reference>
    </ItemGroup>

After building put the resulting .dll into BepInEx\plugins and start the game