Oooh, I love these topics!
I have an outline for the entirety of my comic. It's not super-detailed - it's very much the big-picture view of the story - but it's got a beginning, a middle and an end, with the most important events noted, and when they're supposed to happen. It's just got enough air in between those notes that I've got space to tinker and change stuff that needs to be changed.
To turn all of this from a list of babbling words into an actual comic, I pick a bunch of scenes from the outline (enough to constitute a reasonable chapter, in terms of pacing; I don't set page-limits), and break them down further into what happens on each page - still just in text form. This isn't so much a script as it is a description. Like, "Page 1: main characters arrive in a city. Page two: they run into a pickpocket. Page three; they start chasing the pickpocket" - or something like that. No dialogue, no specific stage-direction.
Then, I take that list, and I brew myself a big pot of tea, pull out my pencil and paper, and get to work turning those vague descriptions into actual storyboards. My storyboards look something like this:
These are storyboards for a Christmas-themed comic I made last year, and each page-layout is about an inch and a half tall - this is to keep me from spending too much time fiddling with details. The text below each thumbnail is the dialogue+some short descriptions of what I've actually drawn, in case I forgot and find it hard to tell. And yes, it is only here that I begin actually writing my dialogue.
The next step is to do a full-sized rough draft with dialogue digitally - I use Manga Studio 4EX for this, because I love its Story-function (which I believe is also available in Manga Studio 5EX). Which looks like this:
These are from a one-shot comic I made a while back.
... and after that, it's off to clean sketches, inking and colouring, and final dialogue-adjustments.
The reason I do the initial storyboards on paper instead of directly in Manga Studio is because storyboarding needs to be a very focused process for me to do it properly - and working digitally is distracting. I mean, the internet is right there. If I sit down with pencil and paper and a pot of tea, I'll only have to deal with distractions from my cat, and those are easier to handle.
As for the groundwork that went into the outline and planning and visual design - I tend to draw turnarounds/lineups for my more important characters, so that I know what they look like from every angle and don't have to make stuff up on the go - and I tend to design floorplans and interior/exterior views of important buildings and settings, too. The less important stuff - random houses along city streets, people passing by in the background, etc., - gets designed on the fly.