@Dmitri-K for his Fixtures Generator. It made it super easy to create all fixtures and results.
And @kingrobbo for his instructions on how to create these files.
1. Extract all files to your editor data folder in C:/Documents/Sports Interactive/Football Manager 2020/editor data
If you don't have that folder create one.
2. When creating a new game make sure to select all editor files that you want
3. Click 'Advanced Options' and add "sourtitoutsi.net Custom Start Date" from the list of nations
You can set it to 'Playable' or 'View-only' - doesn't really matter - but I like to set it to Playable and immediately remove the league after creating the save because I don't want any updates, subscriptions etc on it while playing.
4. For your start date select this:
That's it. Enjoy!
DISCLAIMER: All files now updated to 20.4.0 db.
SI bugged out some leagues with 20.3. and 20.4. db (Brazil, Portugal, Netherlands) which prevented those leagues from being selected so I tried to fix it and it seems to be working right now, but I haven't tested it a few years down the line so if you notice any bugs please report here.