1. To use .NET Core 2.1 with Visual Studio, you'll need Visual Studio 2017 15.7 or newer.
2. Download .NET Core 2.1 SDK at https://www.microsoft.com/net/download/thank-you/dotnet-sdk-2.1.300-windows-x86-installer
3. Install the 32 bit version of .NET Core 2.1 SDK (you can have 32 bits and 64 bits installed side by side) if VS 2017 reports
"The current .NET SDK does not support targeting .NET Core 2.1.  Either target .NET Core 2.0 or lower, or use a version of the .NET SDK that supports .NET Core 2.1."

Add .NET Core 2.1 install instructions.
Add .NET Core 2.1 install instructions.
