Since we were chatting on the other thread, I'll pop in here too!
Getting from that initial concept to the full story/script is definitely super tough, but here's the process I took (which may or may not work for you, since everyone has different work flows xD)
Right from the initial concept (when I had my characters/general idea down) I worked out what I wanted the main 'conflict' of the story to be. The shorter the story, the more important it is that you have this part nailed down, because it kinda sets up the pacing for the whole thing. For example, a comic that's 10 pages long doesn't have as much room for build up as one that's 50 pages.
After figuring out the conflict, I took to bullet pointing out the major events of the story. Very little detail or exposition, but just getting the scenes/plot points down on paper. I started with "beginning", "middle" & "end* and then interpolated from there. Since I knew who the characters were and what I wanted them to accomplish throughout the story already, this was made easier. I knew roughly how the story started and how it was going to end. Filling out the middle was the hard part, and took the most time. But yeah!
Next I took my bullet points and wrote them out as like full paragraphs, in like a loose novel format. I guess this is the closest I got to "scripting" in the whole process, although I pretty much left out all of the dialogue, focusing more on describing how the scenes played out.
Editing down came next. I knew from the outset I wanted the scope of the project to be ±50 pages, and reading back over what I had written I knew I would go wayyyy over that. So I started trimming the fat. I had a bunch of largely unimportant characters, so I cut them out. Removed some of the fluffy in-between scenes that did little to advance the story, etc. All things that would have been at home in a lengthier comic, but had no place in this little one-shot.
Finally, I opted to just storyboard out the whole thing instead of doing a formal script. This took like 2 and a half weeks, but was super valuable for me! I'm more of a visual person than text-based, so it was easier to get a sense for what happened in each panel and who said what when and where by just sketching it out. Further editing down happened in this process too, as I got near the middle of the story I realized it was progressing too slow/taking too many pages so I had to adjust some things on the fly. When all was said and done, the story came out to 66 pages. 16 over my goal, but I didn't feel like editing it down anymore, so that's how it ended up xD
For an example of what my storyboards looked like/quality wise (really quick/sketchy):
But then having that has made drawing the real pages easier since I know roughly the expressions and where the text goes/how much of it, etc. Here's how page 2 finished looks for comparison:
So yeah! Def don't rush it and be sure to take ample time to plan the story out, since it'll make things easier down the line 
part of what's scary about long comics is that a lot of people (myself included) don't do enough of that planning before jumping in. Even just having a general bulleted list of events and then doing the other stuff chapter by chapter is better than going in blind xD if/when I ever get back to my long story, that's probably what I'll do. The planning for this comic was a massive learning tool though!
