Generally, a memoir is based on a few isolated years (or series of events surrounding one subject). If you're writing about childhood, then teens, then adult, it's called autobiography. This was the most difficult thing for me to understand when I started writing my memoir. I wanted to focus on a ten year period, but childhood scenes seemed relevant to this ten year period. So I used flashbacks--carefully though and only when absolutely necessary, because they bring story to a screeching halt. But if used sparingly, and at the right time, they can be quite effective for filling in some gaps. Just don't use ones that are not relevant to the years you're focusing on. For example, if your memoir is about how Grandma's death destroyed the family because they fought over her inheritance, than a flashback about protag watching Grandma volunteer at a soup kitchen is relevant (it shows her generosity) but a flashback about how you won the spelling bee in 3rd grade probably isn't (unless somehow it ties into the family greed or Grandma). If you haven't already, check out the memoir books thread in this forum. There's several books on memoir writing that will help you understand the form. They won't write the book for you, but I found them helpful for generating ideas and narrowing my focus. Good luck!