To be fair, I should point out that the blurbs that appear on the back of nonfiction books and abstracts for articles are usually written only after the book has been completed.
It’s always easier to summarize your main idea when the writing is done.
But as you sit down to begin your work, you need to have in mind a clear and short statement of what it is you are trying to give to your audience.
The best way to approach any writing project is to think of it in parts. You need to start by understanding what it is that you are breaking up.
It’s perfectly okay if your big idea is a tentative. Most writers at the beginning of a project are in a chicken-and-egg situation. You have to have a starting point for organizing your ideas. At the same time, that central idea will develop as you write about it.
You need to start somewhere, though.
Take a minute and write that brief description of your main idea. Actually write down the words.
Remember, three sentences, max. Try for two. Or one.