What did we do (1)?

All we did was

#define 

one identifier and

#include 

one header and we got everything - even an implementation of main() that will respond to command line arguments. Once you have more than one file with unit tests in you'll just need to

#include "catch.hpp" 

and go. Usually it's a good idea to have a dedicated implementation file that just has

#define CATCH_CONFIG_MAIN 
#include "catch.hpp". 

You can also provide your own implementation of main and drive Catch yourself.