How to write your own book, have you had any experience?

My husband has been toying with the idea of writing his own book for quite some time now. We’re actually now discussing how this could one day become a proper manuscript. The only problem is: neither of us has absolutely any experience with this.

It gets tricky right from the start. Do you just start writing, or should the whole story be more or less in place beforehand? Even with a finished manuscript, there’d probably still be plenty of unanswered questions. You read about test readers and editing, then about publishers or self-publishing. Getting help from a service provider also seems to be an option. As a complete novice, it’s easy to lose track of everything quite quickly.

Has anyone here written a book themselves and can tell us how you got started back then? I’d be particularly interested to hear what, in hindsight, you would have done differently with your first book.