The project uses .NET 7 to install, so you need the .NET 7 SDK https://dotnet.microsoft.com/en-us/download/dotnet/7.0
Use Visual Studio 2022 Community, as it's always going to be better than 2019.
I myself use GitHub Desktop, which makes it incredibly easy to manage GitHub projects. Once you've forked the project to your own account/repo (you've already figured this out), open up GitHub Desktop and File->Clone Repository (this will clone it from GitHub's server to your local machine)
Once it is loaded, you can view the files (ctrl-shift-f), which will show the root folder of the code project
We want to open and edit the .sln (solution) file.
With the sln open, you want to update the NuGet code libraries that Pokefilename reuses. Manage...
Install the NuGet for PKHeX.Core to the latest:
With NuGet updated, you can rebuild the entire project to get the outputs.
Since the project is kinda outdated, the methods it uses in PKHeX.Core have been relocated. You'll see the following errors in the bottom pane:
The first error indicates it was unable to build the application (csharp compiler), and the second indicates it was unable to reference a specific method in PKHeX.Core. We need to correct that. `PokeCrypto.GetCHK` was relocated to a different class & method name, namely: `Checksums.Add16`.
With that fixed, we can rebuild the sln again (CTRL-SHIFT-B) and it succeeds (with some warnings)... but the output folder (PokeFilename\PokeFilename.GUI\bin\Debug\net7.0-windows) should have your *GUI.dll plugin file.
Now, you don't need to do the remainder, but here's how you can contribute these fixes/updates back to the owner's repo:
In the GitHub Desktop UI, it'll show you a diff of the files you changed, and the ability to "commit" them to your local repository. Go ahead and "commit" to "master".
Once committed, the commit will only live in your local repository. You can "push" it to the remote host (origin) on GitHub.
You can then create a "Pull Request" to request your commit be integrated into the main repo codebase. Branch -> Create Pull Request
Create a new Pull Request, which will allow you to send it to the owner's repo (whoever you forked it from) and they can accept & merge the pull request.