My writing process is a mess, but it works for me. XD
Step 1: I want to do a thing! Doing a thing would be cool!
The very first seed of an idea appears in my head. Maybe the thought is "scifi is cool but it would be even cooler if it wasn't so full of white people". Maybe the thought is "I really love wandering swordsmen, I should do a story about that sometime" (which, hey, is how I got to Grassblades!)
Step 2: Poke at it
I let it rattle around my head for a while, maybe doodling a picture or two in my sketchbook, but nothing serious. It needs to rattle around for a while to get all the rust knocked off of it, so I can see the coherent idea beyond "swords are cool" or whatever it was I started with. Sometimes, this is a pretty quick process - I can go from "I should do a thing!" to finished thumbnails of the thing in 24 hours, if I'm doing a oneshot shorter than 20 pages - but sometimes, this can take ages. In the case of Grassblades, it took 4 years.
This is where most of the brainstorming gets done - where I develop the personalities of the main characters, where I get a sense of the plot, where the underlying themes start coming together, etc., etc. Also, this is where the research starts, if I feel I need to do any. Collecting visual reference, reading up on things, etc.
Step 3: Plan all the things - and then plan all the things again
Outline the whole plot, scrap 2/3rds of the outline and start over, furiously build worlds, design costumes for the characters and plan the settings and environments, put two things in one scene and see if it works out - and when it doesn't, try again with a different constellation of things. Here's where all that brainstorming and all that sense of the plot start getting structured into some kind of coherent chronological sequence. A happens and then B happens which has to be before C happens, and so on and so forth.
Things are still very vague, though. Like, in my outline for Grassblades, the first 33 pages are summarised as "Masahiro and Akane meet in the woods. He tries to leave her in an orphanage. This fails to work."
Also - additional research done, if necessary. Research is ongoing throughout, since I often don't design secondary characters until I reach the chapter they're in.
Step 4: Page by page breakdown
Take a chunk of the vague outline large enough to constitute a chapter (example in the previous point), then start going through it and deciding how that's going to happen, and what the pages are going to contain. Write a list. "Page 1: Masahiro walks through the woods. Page 2: He spots something weird by the side of the road. Page 3: He goes to investigate. Page 4: He finds a clearing full of dead people." ... and so on and so forth. This helps me decide what a scene is going to look like, and how the story is going to be paced.
Step 5: Thumbnails, thumbnails, thumbnails
Take the page-by-page breakdown, break out the paper, pencil and the best eraser I have (I'm gonna be using it a lot) and start turning these one-sentence descriptions into workable page-thumbnails. They're about two inches tall by one inch wide, and are covered in incomprehensible scribbles and stick-figures - and at least a vague plan of where the speechbubbles go, because hey, this is also the step in which I write the actual dialogue!
Before this step, no dialogue except "Character A talks about [a thing]" or "Character A and Character B argue". With this step, first draft dialogue! It's still pretty vague - a lot of it is literally "blah blah blah" - but it's there to give me a better sense of what the final draft is going to say.
Steps 5-7: Draw the thing
Turn the thumbnails into full-size sketches, write in the first draft dialogue - turning it into second-draft dialogue as I go, tightening things up and turning "blah blah blah" into actual human words - then ink, colour, shade and tweak the art.
Step 8: Proof-read and tweak
When it's time to post the new chapter, go back and proofread the whole thing, pick out the spelling mistakes and the grammar-mishaps, and change my mind about the dialogue a dozen times. How can this be phrased better? Do I really need that line of dialogue or can I do without? And so on.
Step 9: Post the thing
And up it goes online!