Sketch it out.
I have said may times to new artist, sketch. You don't need to be an accomplished draftsperson, just learn to sketch out an Idea. And For me...No, computer/tablet sketching does not count. Here is why. As a physical artist we work with our hands, directly on to the clay. The sketch starts that process. It engages the muscles , that connect to the brain in a visaral way that working on a computer does not. So Use a pencil and a piece of paper! or pen or crayon, but directly on to paper.
The sketch is a road map! It takes the creation out of your brain and makes it real, physically posible.
Allows you to explore the theme, add supports, and flush out the idea.