My process is similar to what @DualDragons described! Usually my stories begin with me thinking about a really cool moment or interaction that I really want to include (usually a climactic fight or battle, in my case, but sometimes also just an overall concept) and then I kinda work around/up to that point.
Usually after coming up with the "big idea" I'll try to figure out how long I want the story to be and go into making bullet pointed lists of all of the major events that will happen as I come up with them. This process can sometimes happen really fast if I've been thinking about the story for a while or am struck with inspiration, or other times will take forever if I get stuck. Having the story length in mind up front helps me roughly figure out how many "events" I can include. If you come up with like 15 events but you're aiming for like 10 pages long, for example, you might run into space issues lol.
Next I start writing out under each bullet point going into more detail on everything that happens both for the event listed, as well as how I'll get from that bullet point to the next one. It's almost like prose, but not really because I write it really sloppily and leave notes to myself within and such. No one else will ever see it, so editing it for proper flow, punctuation, etc. isn't super necessary. Following that I'll go through the whole thing again one or more times for editing. For my comic on Tapas, for example, I ended up needing to omit several characters and a few scenes at this point because the story as written was going to go wayyyyy over the page count I was aiming for, so I grayed those out, reworked the middle part of the story, and wound up with something that was closer to what I was aiming for.
I tend to forgo making an official looking "script" after this step like a lot of people seem to do- I only include key dialogue and phrases that come to mind while writing, but otherwise I come up with most of the dialogue on the fly while I'm drawing out the storyboards, which I do here in lieu of a script.
Aaaaand that's pretty much it. I storyboard out the whole thing, and then get into the production of the comic~