You would need MS Visual studio. The steps are the same as you would clone any project from Github.
The advantage with NetSim is that you can import it as a 'workspace'. That way the code you build will directly run in NetSim. You dont need to replace DLLs etc.
The secure URL for Github repository provided in the project documentation available in the file exchange can be used to clone the project via Visual Studio(VS).
Open Visual Studio and go to 'Team Explorer' window. Then we go to the clone tab and paste the repository Dll. Upon cloning, a local copy of the project gets created.