When it comes to planning out stories, writers seem to fall anywhere between two ends of a spectrum. Towards one end, there are the people who write down detailed scene-by-scene outlines of their stories, often with character biographies and setting/world-building notes as part of the package. Closer to the other end, you have the writers who prefer to “write on the seats of their pants” (hence the term “pantsers”), eschewing planning altogether and not necessarily knowing what’s going to happen in the next scene. Although both the planners and pantsers have plenty of published writers in their ranks, in my experience it’s usually the planners who are most convinced that their method is ideal. Certainly most books on writing (with the distinct exception of Stephen King’s On Writing) seem to advocate the planning approach.
I personally would say that individual writers should go with whatever works best for them. As for myself, I’ve found that I lie somewhere in the middle of the spectrum, albeit maybe a bit closer to the “pantser” side. I absolutely do need some idea of where a story could go when I start writing, but I don’t necessarily have to write a whole outline down. In fact, every time I’ve actually finished a story, most or all the planning I did was in my head. By contrast, whenever I try to write a complete outline for a story, I find myself blocked sooner or later. I am still not sure why that is.
To be sure, all my successes as a writer have been with short stories. I have yet to complete an entire novel. It could be that novels, since they naturally have more going on, require more meticulous planning than shorts. All that I can vouch for right now is that when it comes to short stories, I seem to favor a sort of mental planning that almost looks like pantsing.