I have no idea how to write well. It's probably just a matter of practice and reading good books. But I can tell you the method I have developed for sitting down at a keyboard and actually writing a story.
Note: this method should be judged by its purpose, which is to get an idea from your head into a story rapidly, enjoyably, and fairly well. I use this to write short stories (around 2000 to 5000 words long); I have no clue how well this would work for a novel. I have tried to write longer works, but they always grind to a halt. Additionally, I mostly use this for a very specific genre (weird fiction/paranormal? stuff bordering on horror, with a mild psychological/artsy element), so I have no clue if it works outside of it. But if you want to write decent weird fiction short stories quickly and efficiently, here's your guide.
The secret is flow. If you are EVER sitting in front of your keyboard unsure of what to write, don't. If you don't know what to write next, don't. That way lies stagnation and the spiral of writer's block.
Of course, this is easier said than done. Here are the things I've devised to help me do so, in no particular order.
1. Make up some COOL scenes ahead of time. You have a story idea already— if not, get out of here and come up with one! Once you've done that, figure out what cool scenes you can pull out from it. Milk it for all that it's worth. Dramatic choices that characters must make, incredible things they must react to, and so on. Think of plot twists, and then make sure that they will actually be satisfying and interesting and not just a rug pull. You have been granted an idea; your job is to express that idea, that plot, that monster or alien or situation or murder or obsession or romance or character, as best you can. Find what makes it unique, and use it. If dealing with some element outside of ordinary life (the paranormal, the science-fictional, or the merely exotic) figure out the best, most interesting way to describe it. Don't waste cool stuff. If you are only writing sentences and situations that have been written before, why are you doing this? You don't have to be wholly original— but if you've got something original, show it. And if you don't have something original, show the parts of it that no-one has ever shown before. It's not that there aren't plenty of ideas— it's that the idea has to be used well for it to be enjoyable.
2. An outline is your friend. An outline is the roadmap which you will follow to tell you what to do next. You don't need to record the connective tissue, just the bare-bones outlines, the central scenes. Whenever you don't know what to do, look at your outline and pick the scene that most resonates with you, that you haven't written yet, and write it. Oh, and by the way— the scenes on the outline should be scenes you have a good idea of how to write. Ideas that you don't know how to write should be off to the side, so to speak. And try to figure out how to write them as quickly as you can.
3. If you don’t know what to write here, move on. Never waste time sitting in front of your computer screen when you ought to be writing. Don't know what comes next? Don't write it. Move to the next scene, leaving a gap if you have to, come up with a new scene and write that, just do ANYTHING other than sit there and wait.
4. Parallelism is your friend. Plan out symmetries and dramatic repetitions in your stories. If you don't know what to write, go back and figure out how to put a new spin on old words. Echoes, irony, all that. You can even add this retroactively, reading over your old stuff and using it for information. Just don't be boring about it.
5. Foreshadowing is your friend. Go back and add it if you need to, or retroactively use details already written as such. It's both dramatically satisfying and a good way to keep writing and keep the flow.
6. If you can’t move on, move back. Read over what you've written, and don't just look for parallelism and foreshadowing opportunities— edit! This is such a loathsome, dreadful task that it makes writing look better by comparison— so it's a good motivator to keep writing and keep flowing. And if you can't keep writing, then you're stuck having to actually do it. Which will help, by the way. Oh, also? When you're finally done, read your story aloud. I sometimes get lazy and don't do this, and it shows.