I started by opening Visual Studio for MAC and starting a new project.
But there was no option to create a new ASP.NET core project. There was an option to create an ASP.NET MVC Project but that is using the .NET 4.5 framework which I don’t want to use for this project. So what I want to do first is to get Visual Studio updated to support .NET core. Microsoft provide help on how to set up .NET core on MacOS here: https://docs.microsoft.com/en-us/dotnet/core/tutorials/using-on-mac-vs.
So I downloaded and installed the .NET core 2.1 runtime https://www.microsoft.com/net/download/macos.