There’s a helpful online tool that guides you through structuring and writing an essay. Maybe that will help?
http://www.readwritethink.org/files/resources/interactives/essaymap/
I’ve also found writing every idea on a separate post-it note, then being able to move them around and group them or sequence them is really helpful too.
More strategies:
Penny
ADDitude Community Moderator, Parenting ADHD Coach & Author, Mom to teen w/ ADHD, LDs, and autism