@PotomacBobMost story ideas come out of the blue. Some come fully-formed, which is awesome and convenient, but most are just a clever setup or situation that could be developed into a full blown story with more consideration. Because I write a lot of older man/teen girl stories, the circumstances that bring the two main characters together is critical. I strive for plausibility, so I spend a lot of time contemplating how to make that happen in a believable way. Those contemplations often form the basis of the stories you see published here.
I generally have an ending in mind when I start to write, but not always. Sometimes, if it's just a short, situational story, I'm happy to let it develop naturally, nudging it here or there if it starts going off in wild directions, but surprising myself with whatever ending the story demands.
More recently, my stories have become more intricately plotted, and I've begun to outline my stories to establish the major story beats. Hook, inciting even, midpoint, climax, etc., and might even fill in some of the periods in between if I know I need a certain thing to happen at a certain point. I've become more concerned with story structure and pacing, and now find my stories are shorter and the plots tighter.
After writing the opening one or two chapters, I decide if the story is worth continuing. If it is, then that's when I start sketching character profiles and doing research. As mentioned earlier, I strive for believability, so getting the details right are important to me. Even on shorter works, if I'm writing on a topic about which I'm not well-informed, and it forms an important aspect of the plot, then I'll take the time to learn about it so that it reads as believable. In my opinion, such attention to detail can help elevate a mediocre story to a good one, and a good story to a great one, especially if the reader learns something new and interesting.
By the midpoint, I've settled on the ending. I won't continue writing until I have the ending firmly settled, as all the setup in the first half needs to pay off in the second. This is also where I might go back and pencil in some foreshadowing and set hooks for later payoff.
My big weakness is editing. I edit as I write, then do a final read-through at the end, then I publish. I may go back after a short time to refine the editing with one more pass, but not always and probably to my detriment. My guess would be that my haste to publish probably ends up costing me a half point or so from readers irked by typos and grammatical issues.