I've started a video series on YouTube showing the process I use for creating fictional leagues. I currently run a fictional/historical league (Figment League Baseball, link in my sig) that's in it's 67th season (the first 50 were solo, and we're currently entering our 17th season as an online league).
I'll cover the basics and also in-depth stuff like editing the data files to have custom cities, populations, names, schools, etc.
If you're interested in watching, here's the link:
OOTP Baseball 23 - Creating a fictional league for solo or online play - part 1
I'll update this thread with future videos in the series. If you have questions or feedback, feel free to post it here. I hope anyone looking to do fictional leagues finds it informative - I love fictional play & hope you will too!