Since I recently started modding, I will try to explain in short and short sentences what is important for it.
First before, I have to say: YOU CANNOT INSERT NEW CONTENT! REPLACE EXISTING CONTENT ONLY! THIS APPLIES TO LEAGUES, TEAMS, PLAYERS AND GRAPHICS!
Via the Frosty Editor you get to the CompData folder…. There save the files:
Here you regulate the structures of the tournaments and competitions from FIFA. Inserting a complete new tournament is not possible, but replacing an existing one is definitely possible.
Please note that if you add an additional round to a cup, you must also change all other competitions, `cause all stages, rounds and more are numbered here. It’s important for do not lose their connection to each other ... is a very complicated story.
For example, I inserted an additional main round with all 3rd league teams in the DFB Cup (Germany) because teams from the 3rd league only qualified or inserted by chance. So just look at the structure and try to understand!
Here you assign the competitions, including all their stations, to the teams.
If you replace an existing league and, for example, assign it to a league association (promotion and relegation), it is of course important to create an existing game plan or calendar.
Once you've gotten into it, that's actually one of the easiest changes.
All competitions and tournaments are managed here. Promotion and relegation, prize money, relegation markings, table sorting, start dates and and and.
Important for FIFA to know what to do with the leagues and teams before and after a season.
Self-explanatory ... here you should choose an existing weather system for the appropriate region of the league.
Hope I could clarify a little more. Have fun and success with modding.