Your idea is probably just a bare bones sort of thing, but it must begin
somewhere with
someone doing or discovering
something that alters their 'normal' routine, whatever that is or was and sends them off on the story line.
Story question? That is the unwritten question placed fairly quickly in the reader's mind and the reason he reads on - to find the answer to it. Will the main character find, achieve, resolve, defeat, overcome.....or whatever verb applies to your story situation.
From there on in it's one step after another - with the character moving ever closer (despite setbacks) to, and finally resolving, whatever the story question is.
Take it from there -there's not much else that can help you, I'm afraid. From there on, it's write and see what happens, remembering all the books you've read and how things developed in them.
You can make initial notes and/or plan till the cows come home or you can just plunge in - do whatever works for you. There is no magic lantern step-by-step solution.
Good luck.
I have come up with what I like to think may be a startling new idea for a story. Well, at least a new combination of cobbling some old ideas together.
But I’ve never really learned any step-by-step process by which an idea is expanded to a written story.
Can someone explain this to me? Where do I start? What do I do next? And so on.
Thank you.