You need to set limitations in some way first. You don't make a school of athens from just random scribble. You need to have a prompt, a theme, subjects, and schemes to guide you into creating rough drafts and then you can refine from there. Do research into the subjects. Write down ideas.
I mostly design decorative things, but at least I know what object I will be decorating and what material I am working with. Then I look at various things, do sketches, combine ideas, appropriate designs towards my end. It gets to the point that I can close my eyes that a kaleidoscope of images flash in my mind's eye. Drawas much of it out because it is often an impression. This can happen for anything like poses, faces, hairstyles, clothes, icons, architecture, letter forms, sculptures. Like seeing tetris after playing it for some time.
Don't reply if you have aphantasia.
