Lol - so many answers. Here I'll tell you mine too for Aetherwar.
First phase: Brainstorming
This is the most exciting phase. Write down all your characters. Create bios for them. Sketch them. Think of interesting conversations they have. Think of epic adventures they have etc. I guess the important thing is finding some way of recording things down. Don't worry about what you think of. Nothing here is set in stone.
By the way, I have spent long periods just in this phase. Months. Years... Not recommended for consistently producing comics. But no matter what you need some good time spent in this phase.
Second phase: Outline
I don't create an extremely detailed story. But I do make sure the backbone of the story is strong. This is like the main flow of events. Does it make sense? Does it engage? What are the vital elements that if you miss the story makes no sense or is weakened? Answer the IMPORTANT questions. What is the theme of the comic? Meaning what is the main question it is trying to ask? What are your hook points? What's unique about this story?
For Aetherwar, this was a three week process. I had one pretty intense script and threw the whole thing out just because I felt I was introducing too many elements and in fact that story needed a prequel: the current one I'm drawing. 7 iterations later I found I was a lot closer to what I wanted.
Phase 3: Making it into a comic.
Okay. Some people like thumbnailing the ENTIRE comic first. And when they are happy with it they ink and colour the whole thing.
For me I prefer going a scene at a time constantly referring to my backbone i produced in the last phase. Why? Because I have this weird thing where... well believe it or not, the characters start taking a life of their own. It's really odd. I'm panelling out a scene and the two characters suddenly start saying things to one another that I didn't plan for but just feel right. I'm often left blinking at the screen wondering why the heck I just typed out what I did. A secondary reason is that scenes can also take on a life of their own. A sudden demand for quiet or a small cute funny shot.
The problem with this is sometimes a single scene can demand your entire backbone to change thus you risk creating a plot hole or lose your original theme. However, because of the life and uniqueness this brings to the characters and because I feel people are always more engaged by interesting people doing boring things than boring people doing interesting things, I believe it is worth it.
Well, I don't think my method is particularly better than anyone else's and I see a lot of good advice already. For whatever it is worth though, I hope it helps! It's a long road, so get started! At the end of the day, we all learn best by actually giving things a shot.