Many people simply dictate their stories to a ghostwriter over the phone or in person. This is an important part of the memoir process and helps a ghostwriter to capture a person's narrative voice--the cadence and rhythms of speech that make the story authentic and compelling, as well as the emotional content of a story or the client's unique personality.
It's preferable, however, to jot down your thoughts and recollections in preparation for writing your memoir before you even contact a ghost. It's not mandatory, of course, nor do your notes have to be complete. They can be snippets or bullet points of what you want to cover or what you believe to be most important about your life.
You don't have to create a formal document, merely write or type what comes to mind. It doesn't have to be grammatical or written in paragraphs. The ghostwriter will handle the content and its organization once he gets the necessary information.
Creating source material will help a ghostwriter conduct better interviews and know what questions to ask. Most people relate material in a non-chronological fashion, creating the need at a later date to revise chapters or insert various information or events that were originally omitted. Providing written source material helps frame the narrative structure of a book from its inception.
A little forethought can ensure that your memoir will contain all of the relevant information that has inspired you to write the book in the first place. Just a few minutes a day will give you a leg-up in starting the book you've dreamed of.
The exception is using long, technical research documents or court testimony. People on the witness stand don't speak in grammatical sentences, and readers don't want to plow through a hundred pages of boring, unintelligible statements. Mainstream publishers do not use court testimony.
You may also want to read Memoir Inspiration.