Try an idea board first, (go to craft store and buy a poster board and marker or crayon) or a big art tablet of paper.
Divide the board into nine or twelve squares and number them, then fill in the overall events in a few brief words with the whole story.
Do another board with main characters, and their backgrounds.
Then, as you get flashes of ideas fill in details in any order they come. You might have boxes
and can add in supporting details to each as ideas come.
Then type it all in a computer word program with the nine or twelve chapters as you have it on the idea board.
You can change it as you go along. But now, at least you will have main characters, villains, heroes, twists, plots, surprises, mysteries and endings.
Let it develop over the years, take your time. Leave room for sequels, or miraculous recoveries for the future.