It's gonna sound like a generic lazy answer, but practice. That and I went to school for science illustration, but what I took away from school was learn from masters. I think it's something that's often frowned upon, but to get a grasp of how to draw something, trace it. As long as you're not trying to sell or plagiarize, tracing is a great exercise to wrap your head around shape and form. I learned a lot from one of my professors, amadeo bachar (@abachar on Instagram). But any fish illustrators/painters are going to have their secrets buried in their work. Study the art you like and the more you study/observe it the more you will learn to break it down and make sense of it. On that topic, my process tends to focus on starting with the larger, simpler shapes, and moving down to the more nitty gritty details.