Help with creating a mod (Stellaris)

The essence of this. I am for the new game made a large number of civilizations, based on his book, but did not want to simply play with them, and to set the maximum size of the Universe, then ask the border (that is, stars, the initial fleet, and so on) all of these civilizations that you can select the start placement - like campaign in Victoria 2, HoI 4 - because there you can select multiple campaigns, different.
Now, I understand, to do this mod. Can anyone suggest how to do this? Those civilizations I've done without using any mods, just using the tools to create a civilization