Guide for Adding Leagues in Pregame Editor

I'm very new to playing FM, just started a few months ago.  But I've been playing similar games like Out of the Park Baseball for decades.  I want to add American leagues (USL, NISA, NPSL, etc), and I know it's possible.  I've looked in the pregame editor and see that they're in the DB.  I just haven't a clue how to make them playable.

I've been looking for a few days for a tutorial on how to add leagues via the pregame editor, but to no avail.

I'm sure there are probably a hundred of them, and I'm sure some of the YTers I watch have done them.  I may just be too thick to find them.  Can someone link me, please & thank you.