Give yourself permission to write a rough first draft. Write pages and pages in which you describe the who, the what, the where and the when of the story. Later, as you rework the piece, the why will be written in.
If you are one of those memoir writers who is not otherwise a writer and who will perhaps never write anything else, know that you need to be kind to yourself. In the Turning Memories Into Memoirs workshops, I am often surprised—and dismayed—at how demanding writers are on themselves at an early stage of the process. There are even times when a writer will not turn in a piece of writing because it was not “good enough”—and that in spite of my having told the group that the writing they would submit would still be in its first draft stage.
Think of the first draft of writing as “fixing” the story in the same way that in days when photographs were fixed by chemicals that stage was important if the image was not to be lost. Your first draft is the stage when you “fix” your story, keep it from being lost rather than make it into a masterpiece.