There are online essay mapping tools, like this one: http://www.readwritethink.org/files/resources/interactives/essaymap/. Mapping out the structure first really helps.
When I was writing my first book, I had each topic/idea/story on an individual post-it. Then I used the sliding glass door to keep grouping them and moving them around until I had some structure and flow that made sense.
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism