Install the .NET SDK, which includes the Runtime and the dotnet command.
[Windows only] Logout or restart Windows to allow changes to %PATH% to take effect.
[macOS only] To avoid seeing "Some projects have trouble loading. Please review the output for more details", make sure to install the latest stable Mono release.
Install Build Tools for Visual Studio (Windows only)
The C# extension no longer ships with Microsoft Build Tools so they must be installed manually.