This is what happens: a story idea enters your head. You develop it— either by thinking about it, making stories in your head about it, or by writing down various details in a notebook. Before long you’ve got loads of material that need sorting.
The first question: is your material going to be expressed in a short story or novella? A novel? Several novels? It helps to have a general idea. If you have enough material for a seven-novel series, it won’t fit into a short story. Do you like the material a lot? You might be able to go for several novels on it. If it’s just a random idea, you might trim down the content to make it fit in a novella.
If you have more-than-one-novel’s worth: which story pieces would work for the first novel? And by first, I don’t mean chronologically first. You may have ideas that would work better as a prequel novel, after a first novel in the series is published.
I have one idea I’m working with. Some of the ideas I came up for backstory are too interesting— for me at least— to leave in the past, but they don’t work well for a Volume One of the series. So if I write from a better Volume One point, I can come back someday and write prequel. If I think it’s a good idea at the time.
Now, I am sure there are people who have ideas that march in an ordered fashion out of their heads. I’m the kind of person that creates a story-beginning and then marches backward into backstory, or forward into a youthful character’s old age. I create more story-pieces than I need, and am not organized enough to sort them out easily.
I use certain books to help me organize my ideas lately. Two are ‘Structuring your Novel Workbook’ and ‘Outlining your Novel Workbook’ by K.M. Weiland. (I also have ‘Structuring your Novel’ and ‘Outlining your Novel’ by the same author.)
Both of the workbooks have a lot of questions to answer about your story. It helps remind you of ideas you’ve already had, so you can write them down. And it reminds you of things you might have to create, such as backstory.
Now, you can change, omit or add questions. If you are writing sci-fi, fantasy or a historical, it doesn’t do to answer questions that assume that every character came from a normal American family and went to a normal/horrible American public high school. Your character’s family background and formal education, if any, may be wildly different. Maybe your character is a space alien who was abandoned by his family at age 9 to live on the street because ALL male children in his culture are abandoned at that age. Maybe your character was taught to read by her mother because all the schools in her culture don’t admit peasant children.
Here is one idea I’ve had. When you read a question about a character, don’t answer in your own voice. Let your character answer it. That’s one way to keep yourself from becoming great at writing planning material and unable to actually write the novel. You can include actions of the character in the answer. Such as, “Peter just looked sad at the question and buried his head in his hands.” or “Amy responded to the question by cursing and throwing her beer mug at the questioner.”