I have read several biographies and autobiographies, but I haven’t read many memoirs (if any?) In my vague understanding, a memoir is an author’s memories of a specific event or time period. Sort of a partial autobiography?
Anyway, I would think a memoir would require picking an event or time period, perhaps giving background of your state of mind before the event, your memories and experiences of the event or time period, and how it affected you.
So the event or time could be the focus (like My Time At Woodstock) or you could be the focus (How My Trip To Asia Changed The Way I Think About Life.)
Those are my clueless thoughts on the matter. For what they are worth. Sorry, I can’t be more help.