In the context of the linked article, take the first example and modify it, by reading a number at runtime with std::cin and call factorial or fibonacci with this number as an argument.
Something like this https://godbolt.org/g/CT5M6f
A compile time debugger for the CTFE interpreter is on our extended feature list for the current interpreter rewrite, but it's less of a priority atm.
I prefer to unit test my constexpr logic with a bunch of static assertions in a cpp file. You could put them in with the code being debugged, but having them separate keeps the header files lighter weight with no real downside assuming you have CI or something set up.
So you can use regular tests/debugging to debug your function.
that's up to the compiler actually. For instance, in debug mode visual studio skips constexpr and does everything at runtime (which sucks if you depend on stuff being constexpr).