I used to watch Project Runway. I stopped watching because I pretty much never agreed with the judges on what was the best or worst look of the night.
Occasionally someone would make a strapless dress and it usually ended in disaster. Why? Because the designer hadn't taken care in creating the underlying structure. I distinctly recall one of the judges complimenting one of the successfully made strapless dresses for its underlying dress.
Basically, what had to happen in order for the dress to be made successfully, to where it looked well-done and fit the model well, was that the designer had to make their dress twice. First in muslin and other fabrics and second with the actual fabric they chose out of the fabric store.
What does this all have to do with writing? With each book I write I have to discover just how much underlying structure I need prior to beginning the first draft. Depending on the genre and sub-genre it can vary widely, for me. With Mirror, Mirror I spent a week and a half building the world it's set in. Histories, maps, ruler lists, character sketches, magic systems, et al. Eventually, I needed to upgrade the project binder to a 1 1/2" size.
But then with a couple of the romance books I've written, I just started with an idea then picked names and researched places as I needed them. These are the worst books in the spectrum of the five under my belt at this point. I'm not saying Mirror, Mirror is my best work yet. I would hope so since it's the most recently written and I like to think I improve book to book. But it was the one I had the most structure for prior to writing the first word.
All this to say, I suppose, that I'm a pantser-plotter and proud of it.