It might seem strange to some that I would recommend outlining your memoir before diving in. I do agree that it would be easier to just jump in and start and see what one remembers in a stream of consciousness sort of approach. Sometimes the best memoirs are not in chronological order.
What is key before you start is to know what the prime ideas are to focus on in approaching a memoir to communicate to your audience. By audience, I do not mean publication per se. This may even be details that you want to make sure are passed on to family members.
Even though it may be fun to discover details as you go, the worst part would be to lose sight of the heart of the matter. Though added and discovered tales may help make the narrative a richer and greater exploration, to lose sight of what inspired to to begin on the path of working on a memoir should not be lost. There will be a point where you are unable to add anything more.