It took years of working at writing before I began to feel somewhat competent at it, so I imagine it would be the same for learning how to design a good book cover. I don't have that kind of time, nor do I have the patience. So all of my efforts so far to design my own book cover and potentially self-publish my first novel The Shard, are not going so well.
Here are a couple of the many samples I have done:
Artwork by András Orr
Art by Shane Tyree
To my amateur eye they don't look terrible, but they do look a bit amateurish also. Even worse, when you reduce them to the size they might appear on sites like Amazon, they don't have nearly the clarity that you see on other such covers that you see. Everything I read says the text must be clear and easy to see when reduced to Amazon size. Sigh, I just don't have this kind of expertise, and I'm not yet ready to break down and pay a bunch of money to a professional!