Here’s a quick guide to get you debugging your Fable app in Visual Studio:

• First up, double-check that your Fable project is set up correctly in Visual Studio.

• Next, simply click on the left margin next to the line of code where you want to hit pause. Just like that, you'll have set your breakpoints.

• Then, hit F5. This will start debugging—your app will launch and the debugger will attach itself.

• Lastly, use Visual Studio's debugging tools to inspect variables,
  watch expressions, and check out the call stack.

Need more help? Check out the detailed Microsoft Learn documentation on debugging in Visual Studio.