A memoir does NOT have to be strictly linear, everything laid out neatly in chronological order.
Jot down, or record, what occurs to you, when it occurs. Even if it seems unrelated to something else you thought you'd focus on. Sometimes the best ideas show up like pieces of a jigsaw puzzle when you don't know exactly where they fit.
Keep a pen and notebook by your bed, in case you wake up thinking about something you might want to include.
Check old snail mail letters if there might be important memories, fresher memories, in those. Ditto for any email you saved.
Check old social media posts or DU posts, if you're one of the DUers who likes to post personal stuff (apologies for not knowing; I don't post much personal stuff and can't keep track of who does).
Talk to relatives and old friends about any events you might want to include.
Check old photos and videos for memories they'll spark.
Just gather a LOT of stuff, and let that suggest how it should be organized. It's possible any editor you deal with, if you don't self-publish, will want it organized differently. But that could happen no matter which book's advice you follow.
Just get the recollections on paper, and then start sculpting.