You begin writing a memoir for a purpose. There is a story you're compelled to tell, whether it is from outrage or wonder or a desire to give hope to others.
If you're lucky, this purpose gives your memoir a beginning and an ending, between which the major events unfold. A memoir is not the story of your whole life--it's just one of your stories. It might be the year you traveled to Nepal to find your spiritual goal, it might be the three years when your mother lay dying.
Then there are all the vivid details of your life at that time. It's important to give some of these, or else you seem an anonymous figure. It's equally important to choose a few aspects, a few other characters, a few incidents. You may need, as I did, to write out a lot more than you can realistically include, and then trim, trim, trim.
There's the back-story. Yes, the memoir is about the years when you mother was dying, but incidents from the past may illustrate the meaning of what's happening now. Most times, it's more effective to jump into the storyline first, showing the writer's dilemma as the mother is dying, and then find ways to weave in the older material through flashbacks or conversations.
And then you weave all this together into a shape that suits your particular story. It might be linear in time, it might be fragmented, it might start out one place and then loop back to satisfy the reader's curiosity about the narrator. "Form follows function" is what the biologists call this. Your story, like a living organism, is shaped by the purpose it serves.
(In Part I of Elements of Memoir, I described Judith Barrington's thoughts on memoir writing.)