I don't think you can learn modding, just like that. Also, I think theory changes. I mean I have hunches about where modding is going, which is against popular theory.
If you want to mod, just google it. "What mods for c3po swgoh.gg?" It will tell you about sets and primaries and what would be good for secondaries. You do that enough times and you'll start to see what you'll need a lot of. You can then farm for these, or look out for them in the mod shop.
Take small steps in remodding some of your characters and from that you'll learn things.
To be honest, looking at a new character that comes and knowing what to put on them....................isn't easy in most cases. I think most of us will go off of what other people are doing.