It really depends on your method. I tend to have an idea at first but I'm not one for scripting, I just keep notes of important details then I make thumbnails of the whole chapter in my sketchbook to study the flow and leave little notes of clues I want to remember for when people do the second reading go like "OH THAT'S WHAT THAT WAS". So I gotta keep myself in check for subtle clues.
but anyway for Cosmic Fish this is my process more of less
Thumbnails of the prologue in chapter 2 plus the first 2 pages. (You'll notice how there aren't any details it's just trying to get the plot down, then placement of what info I want on each page.
This is the page I want to work in. I get the idea: I want Gallo to land on a roof, looking over at something, spot Acantha stealing something, then a close-up to his face.
But I'm not happy with the placement, so I re-do it. I know the info, now I work on it like a puzzle.
So this is the new thumbnail I did during class. (ignore the scribble, I was falling asleep)
So now that's done I KNOW what I want. Off to the computer!
So I make the sketches (which I deleted the layer, so I'm sorry. Then the lineart and I start working with what I want.
(I establish my base colors since with this comic, I do cel-shading rather than painting. The details are in the backgrounds like you'd see in an animated movie)
This works for me, and I always recommend it but if it doesn't work for you, that's good to. But I always,
always draw backgrounds first and use my sketch as a map to know where I'm going. I can lineart the characters but I won't focus on their shading (and not even flat colors) until my backgrounds and basic lighting is done, it makes coloring SO MUCH easier for me, I recommend it)
Then when I'm done, I color in the characters, add extra lighting where need to be, maybe make an adjustment layer to fix the colors. But i'm always focusing on the form of the comic.
Sometimes I'll cheat on some of the shading or throw in something basic because my focus is how I want to drive the eye.and that's the important part you should keep in mind.
As you can see from the first thumb, it was more of a storyboard. The second one showed what I wanted, cutting between what each character is seeing and I could even take the liberty of not drawing the roof in the first panel/background because it's already implied in the page prior.
I actually adjusted and moved Acantha around a lot to give a slope when she starts from the left side of the first panel and ends on the right side of the last panel. It guides the eye even in awkwarly-cut panels which will naturally go up-to-down.
I specifically picked this page because placement and flow was something I really wanted to focus on this one. So I hope it helps clarify.
As suggestions, I also recommend you check out Batwoman comics, specifically J.H. Williams' works who plays with panels to make maps and even force you to spin the book around to feel just as lost as Batwoman when she's on a maze. There can be a lot of creative ways in working on paneling, and I really REALLY recommend Williams' works as an inspiration of breaking the mold.
See? KINDAAAAA what I was going for or what I hope to accomplish one day with enough practice.
Also take a look at this!
I also recommend Scott McCloud's Making Comics since he explains far better than I.
GOOD LUCK! Hope this helps a bit! : D