Keep a diary or journal. If you already write regularly, good for you. If not, start now and catch up as you can. I keep track of important events in a small notebook divided into months. There I note births, deaths, trips, visitors, when we got pets, etc. No details, just the basics. Travel journals note details of trips, and other notebooks contain notes for the memories which are written primarily on the computer. Part of a notebook could be dedicated to ideas for stories, essays, chapters, articles, or whatever you choose to call your divisions.
Find your optimal writing situation, but beware. I’ve heard of writers who must use a particular pen or pencil, type of paper, place and time, and are unable to write when those requirements can’t be met. More than one writer has constructed the “perfect” writing place, custom built with new furniture, windows placed just so, art work carefully chosen, only to find that he/she couldn’t write a word there and retreated to the kitchen table or laptop in the Lazy Boy. Ideally, away from your optimal writing situation you can jot notes while waiting in the doctor’s office or airport, type on a laptop on the plane or in a motel room or on a picnic table. The impetus of the story should become more important than the where or how.
Start with the easiest parts. That might be the story told over and over around the family table, the most vivid memory, the time you were happiest, the saddest, or the most traumatic experience of your life. I recently read a book saying to start with your first memory and work your way on decade by decade. I say NO! Even if you are writing an autobiography rather than a memoir, start with the most memorable events. My dear father started his life story over and over, trying to get the beginning just right. As a result, we have a record of less than the first quarter of his long eventful life. Also, he was using a word processor he didn’t understand and kept losing his work. Don’t make that mistake. As stated before, use the method most comfortable for you. Someone can transcribe it from tape recording or handwritten later.
Write daily. Some experts stipulate a minimum, like ten minutes, and if you don’t know what to write, scribble something like “I don’t know what to write.” until the time is up. Baloney! Look at your notes about possible stories and choose one to start jotting memories: most embarrassing moment, funniest moment, etc. Freewrite without regard to spelling, punctuation, sentence structure or anything that might hinder getting words on the paper or screen. You can decide later what is worth revising. Of course “write daily” is an aspiration, not always realistic. But you may find that involved in a WIP, your mind is often preoccupied with the story. I’m willing to include that process in “write daily” because I’ve found that all that remembering and mental organizing makes the physical process of words written much faster. Another caveat: only thinking about writing does not result in a manuscript!
As you complete your stories, start considering focus and organization. The classic advice is to start with the different: the day of the horrific diagnosis, the lover leaving, the death, the birth of the child who is different. You can then write flashbacks to fill in details. Lacking an outstanding event, you might choose a story that illustrates your theme. A memoir is not an autobiography, birth, year by year to present (or death, if writing for another), but a period of time or related events. Some writers have recorded more than one memoir. A famous contemporary autobiographical writer, Karl Ove Knausgaard, still a young man, has written an astonishing six hefty volumes so far.
Consider what your “theme” is. It may be simply to preserve memories for family and friends. You might choose, as my parents did, to include some genealogical information before relating their own stories. Memoirs may be focused on conquering illness or substance dependence or forgiving abusive relationships. They may be primarily focused on revenge, like Mommy Dearest or see how great I am, like The Art of the Deal, or poor me, like Wild. Or, more likely, a collection of memories, good and bad, a summary of your life or that of a loved one you’re writing for. Related to theme will be your eventual title especially if you’re writing to attract an audience beyond friends and family. Otherwise, Memories of ____may suffice.