What is the proper way to add more source files(.cpp and .h) to a project?

It’s a basic newbie question but I’m having trouble with it.
I’m using Visual Studio 2019 on Windows 10.

Welcome to the forums, and a belated merry Christmas!

How have you set up your project? Are you using CMake?

A belated merry Christmas to you too!

I set up my project by following the ‘Create New Project’ instructions on the main Urho3D page, where it says ‘Using IDE’ and under it ‘Visual Studio’.
I edited the CMake instructions as it said, built it and it works(I get the spinning mushroom with music).

But whichever way I tried adding more source files(even just empty ones), something just ends up breaking.