I have a very general outline spanning the entire story of Grassblades - from beginning to the very end. And by "very general", I mean my first chapter is summed up as follows - "Masahiro and Akane meet --> they go to town, he leaves her". That turned into 33 pages and got a BUNCH more details filled in and more stuff tacked onto the end once I sat down and actually started working on the chapter - but the outline is good to have, because it allows me to have an idea of where I'm going, and where each scene needs to end up.
But it's also so general that I can adapt it.
I mean, I know who each of the major characters are/will be, and I know where they'll show up in the story, and I know how everything ends. But I have creative freedom when it comes to HOW all of that stuff happens, and I'm free to add scenes or subtract scenes wherever I feel it's necessary.
I then script the story chapter by chapter; I decide which general notes of the outline should go into one single chapter, and then I sit down with a pencil and some paper and I start storyboarding and writing the dialogue together. I really can't write it all as just plain text and THEN do the storyboards; they kind of have to grow together for me.
This process allows me just the right amount of structure to keep me headed in the right direction, and just the right amount of structure to keep me from getting bored AND allowing me to fix things that weren't super-great in my outline (because you KNOW there are gonna be some of those; sometimes, we can't see the problems before you run into them). For example - I realised that I needed my secondary characters to show up a little more often to keep people from forgetting them, so there's a brand new scene in chapter four that doesn't exist in the outline, and there's another coming up in chapter... six, I think? Also, I can do a bunch of foreshadowing of stuff, because I know where we're going.
I also have a buffer of pages, so that I'm VERY far ahead of what's being posted online (55 pages ahead, at last count) - this means that once I finish drawing a chapter, I can go back and tweak and adapt the dialogue one last time, if that's necessary.