Making addons in the most basic term is as easy as your skill, competence, and quality of overall planning makes it. The better your skill in modelling, texturing, and coding, the easier all three will be to you. The more competent you are, by which I really mean how readily you can adapt your skill to adapt to Orbiter's requirements, the easier it will seem.
The better your overall plan, the less likely you'll get halfway through and get stuck. Designing addons is sometimes a long and complicated process, and having a clear and well laid-out plan to refer back to is always helpful when you're lost in the middle and can't see for the code.
But really, it depends on you. How long you spend on it, how much effort and attention you pour into it, how much care you treat it with, all this determines the quality of the final product much more than your actual skill with a compiler or a brush. If you're just looking to whack something together in half an hour, throw it on OHM and inflict it on the community, then there are many other addon developers whose lead you are following.
Conversely, if you want to release a high-quality, revered addon, you need to spend time on it. You need to work at it, make it do what you want it to, essentially craft it into life, and all this takes time and effort (and generally a fair bit of coffee). And when it's done, you need to test it, and break it, go back and fix it, test it some more, rinse and repeat until you can't break it any more, and then let someone else try to break it (beta).
It's probably a somewhat whimsical description, but I doubt if many of the other developers of good-quality addons would disagree with the assessment. Remember that there is no rush to get things out, unless of course you're silly and tell everyone what you're planning on making too early, so you get harassed for release dates every twenty minutes...
to the forum! For add-on development tutorials, take a look at the bottom of the 
