How do people writing their memoirs collect their material? Good question, because few people these days keep diaries or records of what they do on any regular basis. That takes work, too, and who has time?
Some people actually do have records to remind themselves of incidents and events being recalled. There are family photographs, school books, perhaps old newspaper stories, letters and other useful documents. But more often people have to rely on their memory, as elusive and inaccurate as that may be.
Sometimes, too, friends and acquaintances can help with reminders. A personal example: at a high school reunion a year ago I reconnected with some folks I had not seen in decades. We exchanged stories and tales, some of things I had not remembered until mention of them brought them back. Some of us got together again months later and continued the reminiscing, bringing back yet more memories of people, events and details long forgotten.
So a hint. If you're stuck, call an old friend or a relative. They might help kickstart the memory process and, who knows, supply a few missing bits as well.