This appears to be an official Microsoft resource for doing the migration. Summarized below:
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(recommended) Retarget all projects you wish to port to target the .NET Framework 4.7.2 or higher.
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(recommended) Use the .NET Portability Analyzer to analyze your assemblies and see if they’re portable to .NET Core.
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(recommended) Install the .NET API analyzer into your projects to identify APIs throwing PlatformNotSupportedException on some platforms and some other potential compatibility issues.
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Convert all of your packages.config dependencies to the PackageReference format with the conversion tool in Visual Studio.
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Create new projects for .NET Core and copy over source files, or attempt to convert your existing project file with a tool.
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Port your test code.