Once you learn about threads, you can, if you like, apply them to memoir writing. If writing is not your thing, maybe recording conversations with family members would work. All a memoir really is, is a collection of stories with a common theme. The thread you follow to collect the stories may start from the past and come forward. Or it might start from the present and work backwards.
For example, you might ask Grandma what spelling was like in fifth grade. Did she have autocorrect? From there you would work your way forward through her life. Or you might be cooking with her when something goes awry and you remember tears of laughter, you ask her if anything like that ever happened before, and then your mind is running backwards. It doesn’t matter how you get there, as long as you capture the stories in some form. How many times have I heard, “Grandpa used to tell that story over and over. I wish I could remember the details. It was really funny when he told it! But now he’s gone, and I can’t ask him anymore.”