The challenge peculiar to SF: You’re trying to show, not tell. But how do you “show" a world that, to one degree or another, is unknown, unique, and may even operate by physics that are different from the laws your reader subconsciously applies?
How do you find the discipline to keep writing? Especially through patches when, for whatever reason, the material resists you?
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.