In order to work with the designer you will need the following items on your PC:
Visual Studio 2017 or a later edition. The type of your VS installation is irrelevant e.g. The designer also works with the "Community" edition.
.NET desktop development package in selected during the installation of Visual Studio:
The TestResults.io Designer extension which you can download by following these steps:
In Visual Studio go to Tools --> Extensions and Updates
Select Online extensions
Search for "Designer for TestResults.io"
Select the Download button
The TestResults.io extension will be installed automatically upon the next Visual Studio restart
You can verify the successful installation of the TestResults.io designer by accessing the Extension and Updates in Visual Studio and checking if you see the extension in the Installed section: