1. Download FinderOuter release or source and build it yourself (deterministic builds and PGP signature will be added in near future)
2. Download Ubuntu
3. Verify Ubuntu's iso
4. Disconnect network cable (to remain offline)
5. Boot from a DVD or USB
6. Run FinderOuter
...\Src\FinderOuter\bin\Release\netcoreapp3.1\FinderOuter.dll
If you didn't want to download VS. then MSDN has some good posts to get you started on how to use dotnet commands.
https://docs.microsoft.com/en-us/dotnet/core/get-started
An example: https://docs.microsoft.com/en-us/dotnet/core/tutorials/cli-create-console-app
the dotnet build command: https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-build
Basically you'll have to run:
dotnet build FinderOuter.sln --configuration Release --runtime osx-x64
I am trying to optimize ECC (which is what slows things down here) but unfortunately that will take a very long time to be ready.