I have been modding the campaign map for some time now. I edited the .markers file by using the converters that taw made. Find it
here. You probably will need a lot of converters and some compilers (jruby, python, java...). That is why I recommend to start with the basic stuff, build up from there and continue until you go more to the advanced topics. I have seen many trying to create new campaigns and all very encouraged. But after a while, became discouraged, because of the time you need to put in to create a really nice campaign map. I am still learning new things, because it's really extensive what you need to do.
Here is an example what I am building with my new mod and look at the map:
It took me a lot of dedication, effort and time to do this. I kind of understand how everything works (not everything). I started the same as you, I made some new provinces in Japan. Than in later time I added a total new landmass (Korea). Where I made a lof mistakes in how the engine works with campaign map details that can be found in the batte map. Those are tiny details, but hugely important.
That is why I say, continue what your doing and piece by piece you get better.