LE How hard is it to make a custom Magic mod?

[Mortimer]

I know there is a bunch that add some really interesting spells, Midas, Apocalypse, etc

 

But it seems most spell mods feel a little bit ... overboard? intrusive? They just don't entirely mesh well with the feel I'm going for in my game? But on the other hand there are spells in those mods that I like or do something very close to what I'd like with a few tweaks.

 

Is it very tricky to combine certain spells from current mods into a custom one (And is it kosher if its just for personal use?).