Download nuget packages
$ dotnet new bepinex5plugin -n Temp -T net47 -U 2020.3.33
$ cd Temp
$ dotnet add package Valheim.GameLibs -v *-*
$ dotnet build

Run dnSpy
From Valheim/BepInEx/plugins Open:
    FarmGrid.dll
From $HOME/.nuget/packages Open:
    bepinex.baselib/5.4.20/lib/net35/BepInEx.dll
    harmonyx/2.7.0/lib/net45/0Harmony.dll
    unityengine.modules/2020.3.33/lib/net45/*.dll
    valheim.gamelibs/0.202.14-5.0/lib/net40/*.dll
File->Export to Project
    Set directory, rest default

$ cd FarmGrid
$ vi FarmGrid.csproj
Add reference to UnityEngine (similiar to UnityEngine.CoreModule)
$ dotnet build