From Hayao Miyazaki:
From within the confusion of your mind, you start to capture the hazy figure of what you want to express. And then you start to draw. It doesn’t matter if the story isn’t yet complete. The story will follow. Later still the characters take shape. You draw a picture that establishes the underlying tone for a specific world. Of course, what you have drawn will not be your final product. At times, your work may be rejected entirely. When I mentioned earlier that you must have the will to go to any length, this is what I meant. When you draw that first picture, it is only the beginning of an immense journey. This is the start of the preparation stage of the film.
It’s worth reading all here.
Draw many pictures, as many as you can. Eventually a world is created. To create one world means to discard other inconsistent or clashing worlds. If something is very important to you, you can keep it carefully stored in your heart for use at another time. Those who have experienced an outpouring of an amazing number of pictures from inside themselves can feel it. They feel that the fragment of a picture they envisioned, the other trunk of a story that was thrown out while piecing together a narrative, the memory of pining for a girl, the knowledge about a subject gained as they delved deeply into a hobby – all of these play a role and become entwined into one thick strand. The scattered material within you has found its direction and started to flow.