I highly suggest looking at the existing formats and using those if possible as it will save a lot of modding time.
If you do want to go forward with a full custom mod here are the basic instructions:
1) Find Teams.xml from the game files... either in the steam apps folder if you're using steam or in C:\program files\Wolverine Studios. I've also copied one to this ticket just in case.
2) Copy Teams.xml from that location to Documents\Wolverine Studios\DDSPF 2025 (this will tell the game to use your modded copy instead of the default one)
3) Open Teams.xml in notepad
4) Copy a league format (I suggest picking one close to what you're trying to do... if none match the easy ones to start with are like Custom6Team14Game)
5) Paste that copy of the <LeagueFormat> to the top of the file inside the top <LeagueFormats> element
6) Change the Format="" to Format="Custom"
7) Update your format to include 2 conferences, and up to 4 divisions per conference. Each division can have as many teams as you want. Make sure each division has LastYearStandings="" in it and they must be unique. So if you have 16 teams, you must have 1-16 LastYearStandings in that division. See the other formats for examples.
8) After you build that, you'll need to construct a schedule for your league. So if you have 8 teams and you want a 10 game schedule you'll need 40 rows in the schedule.csv file, one per game. The file is a .csv of Week, HomeTeamID, AwayTeamID.
9) When you create your league in game select "Full Custom Format" and then click Schedule File and select your created schedule.csv
That should be it. If you want to add more attributes in to customize things like logos, etc. you can look at the Real World Mod to see a great example of setting those items up. https://www.draftdaysports.com/board/viewtopic.php?f=285&t=37471
If you have other questions I would look at the mod forum on the website or join the discord https://discord.gg/xj6sgNck95