What I do when I have a story idea is write it down. On scratch paper.
If I don't have an opening, I start a page just to list all the stuff that seems to go together but I haven't gotten an outline for yet (or have not gotten far enough on the outline). If I do have an opening, it's time to start the outline. The top half of the first page is the character list. In which I put down the names (or --, ---, etc. if I don't have names) and any significant details that don't naturally flow into the outline. Like physical appearance, character, job, relations to other characters. (And if the story gets long, it gets crowded.)
And then I start the outline itself. Your English teacher would not approve. I list the events as they happen. I number them so that I can go back and put in 12.5 when I realize that something is missing between 12 and 13. (Trying to write down events so that they will not be broken up is an art in itself. I don't always recognize it.)
I do it all in gel pen. In a particular color, specific to the story. And if I don't know which color well enough, I don't know the story well enough to outline it. 'cause I've got to know if a new, bright idea feels right to fit into this story.
And then the outline starts to grow. Sometimes I have a good idea where it is headed. (which is why I need a page to list all that stuff that doesn't have a place in the outline yet.) Sometimes I have to inch along and let it sit on the backburner. Sometimes I have to have a man come through the door with a gun in hand -- metaphorically, which is why I have to know the story, because I have to know what to literally have come through the door.
And sometimes -- sigh. I am just about to finish an outline. I realized where it was going when I realized that a certain character would have to go. Now I think I have to sit down and totally overhaul the outline, because I have to make sure that everything she did in the first gets done. Such is the fun of plotting.