looking for help with what files I need to change

In Civics.eco set EnableAdminUI = True. In Settlements.eco set the last value of SettlementFoundationBaseInfluence high enough to fill the whole planet. (Depends on world size, 1200 is enough for the maximum supported size of 2.56 km²)

Then, place a federation foundation in the world. Target it and execute /fe to force enable it. Open it, remove the check at "Needs Child Settlements". Then you're good to go to just set it up as any other settlement and have a global federation.

Moved to general forums, as this doesn't require modding.