There are dozens of books about how to write memoir and this is just a bare minimum suggestion of what most agree is a solid way to get started. Hopefully, It is also a helpful path to productive discussions in your writers group.
Having decided on your audience (family, children, general, published, unpublished, etc) it is important to narrow the focus of your memoir. Why?
Knowing your focus will help create the structure of the whole. For example, if you choose to write about events in a single decade, the progression of time creates an immediate structure. If you are writing about reflections over a three-day period, those three days provide a structure even if the reflections span decades. Or if you are writing about grief, you may stay focused on a single event while examining others, or if you are writing about an event with on-going consequences, the listing of those consequences may create structure or perhaps the point of view from the different people involved may create structure. Again, knowing the central focus of your memoir will help create structure.
If you are having a hard time figuring out the focus of your memoir, make a list of all the stories you want to write. Then go through the list and highlight those that have characters, places or events in common. See if you can find threads that tie the stories together. Then place those stories in the order of a time-line and see if there is an overall theme to the period. Sometimes that theme can become your focus and will allow you to find more stories that are relevant.
In a writer’s group, talk through the possible themes of your memoir and the resulting structure. To facilitate the discussion, you may want to put together an informal table of contents. This will allow others to be able to question the logic of the progression, note possibly missed areas of interest, or even suggest a whole different approach. It will also push you, as the writer, to narrow the focus even further, find more stories and structure of the whole of your work before you begin writing.