I was working on a simple mod just by using other mods as examples and changing them to do what I wanted, but I decided to learn to code with Java and start from the beginning to make it truly my own. If you know how to code at all, I would say it isn't too hard to make mods, but getting started is one of the most difficult parts (kind of like writing a book). The easiest way is to look at other mods similar to the one that you want and work off of them.