I’m about 75% pantser. I can intuit dim outlines; I usually know more or less where I'll end up but have no idea how to get there.
First draft: In your first draft you get to be Wild Being of the Woods. Write crazy. Stop and start, veer off, break rules, color outside the lines. Let the story discover itself. Later you can figure out what to do with it.
What you respond to in an idea or a prompt—what Ursula K. Le Guin called a “thought experiment”— may be big or small, clearly structural or emotionally subtle. Anything, really.