I started when sorting out DM's family papers which she left in chaos on her demise. My plan originally was just to research the branches of the families and make a document tracing family members for the information of DC and DGC. However, using various websites, I found references in B,M &D, old newspapers, censuses, court records, emigration lists...so much now available online, and so very informative. Her father's family had a fascinating and easily traceable history, and so I started in the 18th century with a military posting in Newfoundland and Boston during the American Revolution, and wrote an historical novel about the wife and family who accompanied the army, the tragedies that befell them , factually true as a skeleton of the story, but I fleshed it out with lots of research and travel, imagination and inspiration. It was a great success, and started me on the second part of the trilogy, the little boy who saw his mama die in childbirth during the siege and his papa seriously wounded at Bunker Hill....how that childhood trauma affected him in later life and I linked it with a scandal in his middle age, when he stabbed and killed his daughter's beau...PTSD before there was such a thing. He believed the young man to be a burglar or a threat to his family. The characters just flow from your fingertips once you get started and have imagined what they look like, their nature, dreams, fears, but guided by the outline of the real story, proved by public documents. Or you can deviate completely. I've just finished Book 6, about WWI, and the nurses who served on Malta. It's been fascinating, absorbing, hugely informative. Much more fun than simply recording Gt Aunt Mildred's basic details. Make her come to life, especially if you have older family who remember her and can give you clues as to her character. Good luck...once you get started, it becomes a total obsession.