Determine who you want to write about and the scope of the memoir. Unlike traditional biographies that cover key points of a person's life from birth to death, memoirs tend to cover a more limited time span. What event or events will your memoir cover and who will be writing about them? "General Custer's Account of Little Big Horn" is a fictional example of an historical memoir. There is no such book, but if there was, it would be written as if General Custer were giving his account of what happened at Little Big Horn. The author would change the historical fact that Custer dies in that battle. This presents an interesting "what if" type story.
Research your historical memoir thoroughly. Since this is fiction, you have the liberty of making up some situations but the overall memoir should stay true to historic fact. An exception is if you aim to write an alternative history, such as in the Custer example above. You also want to learn as much about the character whose voice you take on during the writing of this memoir. Your aim is to find accuracy in that voice, so any research you do to help you toward this end will lend authenticity to your fictional memoir.
Make a time line. This is a good idea to have handy as you write your fictional memoir. A time line helps you keep your memoir in context. As you write, pepper your prose with little details that evoke the spirit of the historical period in which your story is set.
Write your fictional memoir. While you write, become your character. Think about how he or she would feel. You may need to work hard to keep your voice from intruding. This is where your research comes in handy. The memoirs are not about how you feel. In a fictional memoir, the tone of your narration becomes crucial to the success of the story. Write actions and thoughts that you base on historical facts you uncover.
Write to entertain. You're writing a fictional account of a historical person's place in a historical event, but you aren't giving a history lesson. Not consciously, at least. In the course of reading your fictional memoir, your readers may learn something about the period but they shouldn't realize they are.