To build on @rusty217 - you would need:
Year 1 file (make sure to enter start and end years) - 1st div 12 teams, 1 relegated; 2nd div 24 teams, 3 promoted
Year 2 file (make sure to enter start and end years) - 1st division 14 teams, 1 relegated; 2nd div 22 teams, 3 promoted
Year 3 file (make sure to enter start and end years) - 1st division 16 teams, 1 relegated; 2nd div 20 teams, 3 promoted
Year 4 file (just need start year as this is going to be the 'going forward' version) - 1st division 18 teams, however many relegations you want going forward; 2nd div 18 teams, however many promotions you want
Make sure to also set the minimum and maximum number of teams for these competitions in the nation rules - i.e., for div 1, min 12 max 18 and for div 2, min 18 max 24