First, think you know the story. Tell it to yourself several times. Read and re-read old journals going back to 1977 and then read excruciatingly painful old letters and look at faded old photographs that you think, mistakenly, will reveal clues to character.
The only clues to anything lie in your own memory. And if you can't believe that, and though I know it is perfectly true I can't, then you're sunk.
The finer points:
(1) A memoir must be either poignant (Maxine Hong Kingston, Helen Fremont, Mary Karr) or funny (David Sedaris, Augusten Burroughs). Or both.
(2) Try to write poignant. End up with maudlin listen-to-my-tragic tale.
(3) Try for funny. End up with sicko slapstick or worse.
(4) Remember the formulae: "Emotion recollected in tranquility" or "Inspiration counts" or "Write and re-write."
(5) Read The New York Times, write a blog post about not writing, unload the dishwasher, re-load the washing machine, feed the guinea pigs, feed the fish, eat something, close your eyes for a few minutes, oh, it's almost time to pick up a child and then--
(6) Lose your keys. A daily practice. Be sure to lose them in a different place every day, so that it always takes you fifteen minutes to find them, and by the time you do you are out of breath, sweating, and late.
(7) Write in your journal about how you're wondering what the real story is, since you've now told it to yourself and found that when you're downstairs with no access to your computer making macaroni and cheese you know the story but by the time you've rushed upstairs to write it down it's fallen into pieces in your hands, confetti, and the confetti turns into mist and evaporates.
(8) Sit down and write the completely fabricated Science Fiction story about the character being chased by God. In whom you don't believe.
(9) Think about the story more until you suddenly feel very, very sleepy and "have to lie down."
(10) Do I really have to re-live it all to write it? Really?
Follow these principles daily and a sufficient amount of nothing will happen. However, if you think about the topic enough to give yourself nightmares, some version of a story will eventually emerge. I think.