I generally start with a really rough bullet point timeline of events in a word document.
Then I'll go through and write out in several stream-of-consciousness paragraphs in more detail what happens before during and after each bullet point in greater detail. It basically reads like if I were telling a friend out loud how the story goes rather than being any sort of polished prose but for comics it gets the job done~ Notably I don't include much of the dialogue at this phase- if there's a really important line or two or five that jump out at me I'll definitely note them down but usually I craft that later. This is legit just scene to scene, event to event.
After I'm done drafting the whole thing out that way I spend some time on editing- for length, pacing, consistency, etc. This is probably the step that I still need the most practice refining lol. But yeah, making sure everything makes sense, there's not too much filler, the pacing is reasonable, etc. My longest recent project, the 70-page one-shot I have uploaded to Tapas, ended up losing 4 main characters, several scenes, and others edited down when I started realizing at this stage that I was gonna go wayyyy over my page goal (which was 50... still overshot by 20 ).
Then lastly, I skip writing an actual script and go straight into thumbnails and storyboards. I'm not good at trying to guess exactly how many panels should go on a page or how big they should be just from words like that- it's much easier for me to just crudely block things out and shift them around in real time to find something that works. I' end up storyboarding the whole thing out up front as well so that I can look at the layouts big picture and make sure that I don't have a panel ending at an awkward point down the line or whatever. This is also where I generate the bulk of the dialogue- I have a good sense of what the characters are talking about from the outline even if I don't know the exact words at that time so I work that out and try to account for it in the page layout (this is also the sort of stuff that drives panel size obvi that I find hard to do in a traditional script).
As mentioned the above process is for relatively short projects though, the longest of which being 70 pages. I'm currently in the process of outlining a longer, multi-chapter series and have been thinking about things I'll likely do differently. The timeline of events will definitely be the same and series-long, that's the phase I'm in right now.
I'm not sure yet if I'll do the detailed write out of the whole series... but given the content of the series I probably will tbh. So that I can thoroughly edit the whole thing in the next phase.
Storyboarding I'm planning to do 1 chapter at a time.
Another tool that I've brought into the fold too is an excel document to use for sorting things out chapter by chapter. Highlighting cells different colors to represent different types of plot points has helped me to see the whole long thing in context a little easier. Dunno how effective it'll be in the end but definitely had fun trying it out so far~