Not going to go in depth with the written explanation just yet but since 2K17 ( I think) I have sometimes made my own jersey mods from the team designs that people upload to myleague team design section.
I never really see anyone talk about it so I have started a video on it... I may need to make multiple videos to really explain if it is not easily understood.
Doing it this way, you don't need to ask anyone's permission because the files are available to all in your local save files.