I usually start with a general arc in my head that I want the overall story to follow, with a beginning, a middle trend, and an ending. It would be enough for me to write down a synopsis or pitch to someone. From there, I spend a lot of time brainstorming the details: Who the characters are, what events they go through, what kind of world they inhabit. This is also paired with developing the characters in particular: what kinds of people they are, what emotional journey they will go through, things like that.
After all that, I start writing something down: Chapter by chapter. I'm someone who likes to edit as they write, so I'm scrutinizing every action a character takes, every line of dialogue, every scenario that plays out, etc. Sometimes I think the plot in that chapter should go a different way, so I change it. And there may be things that happen in that chapter that will affect things down the line, so I gotta play those scenarios out and judge whether its the direction I want to go with:
When I read novels, they play out like movies in my head. The same happens when I'm writing. So since I'm writing a script to eventually turn into a webcomic, I'm also thinking about what the setting will look like, how I want the camera shots to look, where characters will be on "stage," and lots of other things. This eventually translates into the actual drawing of a webcomic page.
I start with sketching out very basic figures, along with the dialogue lines already pasted into the canvas:
Then I draw out the lines. I also allow myself to take some liberties with the lines, they don't have to adhere to my red-line sketch layer exactly:
Then color:
Then background, shading, and finishing touches. I also rearrange where the dialogue lines are placed to complement each frame better:
I also want to add that throughout this entire process I'm still revising and editing and thinking of new ways I want to take this story. It never stops for me, even during drawing out the panels. There have been a couple times I've thought of some last-minute changes while making the comic pages, and I'm always happier with the story or the art afterwards.
I personally think that my drafting methodology is very messy compared to other writers and creators. But it's how I'm comfortable working, so yeah.