If you’re writing out a list of headings or topics that you want to expand on, leave lots of space between them on the paper and don’t bother to number them until you’re completely finished. This lets you add or even subtract headings and items as your thinking evolves – without running out of space.
Get a notepad and a pen, or pencil. Start writing, and as the quote above says, leave space.
I think better with a pen in my hand. Perhaps this process will work for you, too.