I used to write by the seat of my pants..."lead me where it will", as you've said. I haven't ever taken a creative writing class or anything like that, so all of the writing I've done has been "on the job training".
About a quarter of the way through my first book, I realized that just writing blind wasn't going to cut it for a longer piece. I was spending too much time scratching my head with writer's block, not knowing where I wanted to go next. I'd spend week or two on a single chapter. I wasn't even sure how I wanted it to end.
So, I found an article on the "
snowflake" method of writing, and used that to get through the first book. But the snowflake and I didn't get along well. I didn't like the time spent on all the synopses and things that it calls for. I didn't have an issue holding that information in a more generalized way in my head while working on the actual meat and potatoes of the story.
So, for my second book, I opened an Excel spreadsheet, and laid out the plot by "scenes". Basically a short one-liner telling me what was going to happen in a scene, until I had plotted the book from beginning to end. Then I just "connected the dots", writing out in detail the scenes I had previously outlined until the thing was done. I was able to write a first draft chapter in a day or two instead of a week, because I had a fairly clear road map from the start. People have told me it's much tighter and better paced than the first book.
I also started learning more about the actual craft of writing, the three act structure, that kind of thing, but I mostly still just put the scenes together by what felt right to me.
In both cases, if I came up with a new idea, I just went with it. Berry wasn't even in any of the character notes or outlines that I did for the first book, but once he was in the story, he quickly became an integral part.
For the third book, which I've plotted out and have started already, I spent more attention on the three act structure. I figured out the beginning, midpoint, and ending scenes first, which made it much easier to fill in the gaps and avoid the struggles with the "
sagging middle " that I've had previously. I'm still using an excel spreadsheet with a list of scenes, but I feel like I have a better understanding of the basic principles of storytelling. I was able to plot it out in a day rather than struggling with the outline over weeks. I think/hope that this one will end up my strongest work yet, and get completed faster than the 2nd one. I'm pretty excited about it.
All that said, there are plenty of authors who write awesome stories just by the seats of their pants. There's no hard and fast rule that says you have to plot out in advance, or that you have to develop character backgrounds, detailed world notes, or anything of that nature.