Read novels by authors who have based fiction on true stories. Best-selling author Jodi Picoult often uses true stories in her novels. Matthew Bondurant wrote a book titled, "The Wettest County in the World: A Novel Based on a True Story." There are many more examples out there. The more novels you read, the more you'll absorb.
Find a true story that interests you. You might find an article in the newspaper. Maybe a friend will tell you a story you keep thinking about. Perhaps your family history will inspire you. Jot down notes about the story.
Think about the theme. Why does this particular true story interest you? What do you have to say about it? Are you interested in revenge? Love? Mother-daughter relations? Write down notes and ideas about the theme. This may be where the title comes from.
Explore the characters. Even if you feel you know who the characters are, write up descriptions. Do character studies. Part of writing a novel is adding your own insights into characters.
Begin thinking about scenes. What are the dramatic moments in this true story that you will want to write about? A trial scene? A romantic dinner at which a man proposes? An arrival at Ellis Island? Consider details you'll use in the scenes. How will the characters talk? What will they wear?
Start writing. You should have enough material by now to fill up pages. But be patient. Writing a novel can be a long journey.