Creating an illustration requires working knowledge of things like perspective, anatomy, gesture, lighting, color theory, etc. And all this knowledge does not guarantee a pleasing or satisfactory finish.
However there is the process of thumbnailing, prior to the final illustration that can guide you in the best direction. This process is a mandatory prerequisite for a successful illustration process. It again does not guarantee that you will love the final result, but it is definitely the smartest way for going about making an illustration. Why?
In these small 1 or 2 minute sketches, you can explore a multitude of compositional ideations. When you find the one sketch that solves the most problems then you can move onto the final illustration. Do not be afraid to fill up pages of thumbnails prior to setting off on the final. This will only serve to improve your final with the best starting point.