Obviously the first thing you need is an idea, but I can't really help with that aspect, so I'll skip right to the next step. Once you have your idea, you need to break it down into issues. Even if you are planning an on-going series, you should at least do this for your first arc. The number of issues is really up to you, but I've found that a standard length for a single story is 4-6. Next, I've found that listing each issue helps to lay out the plot. To do this, I write up the following in a notepad:
You repeat it for as many issues as you feel you need, though you can always add or delete an issue if necessary. The "Plot" is just a one-line description of what will happen in the issue, while the "Hook" is how the issue will end--the "I need to know what happens next" moment for the reader.
Once you are happy with the overall story progression, you need to flesh out the first issue. I do this by simply writing a list of 1 to 22 in a notepad and writing a single line for what will happen on that page. A good idea, I've found, is to draw a line between double-pages so you can visualise the layout of the comic, like this:
Pages 1 and 22 will always be on their own, but the rest will be paired. This comes in useful if you want a shock reveal, as it will have more of an effect if placed on an even number because the reader will only discover it after flipping the page. Another benefit of this layout is for the placement of double-page spreads.
Once you have each page of the issue planned out, it's time to start writing them. This is where my method differs from that of other writers I know. Because I'm a very visual person, I like to sketch my layouts before writing them. This gives me a rough idea of what the page will look like before I put the time into dialogue or description. Here is an example from a project I'm currently working on:
As you can see, it's nothing overly detailed, just the layout of panels and positions of objects and speech-bubbles within them. From this, I then begin writing. There are many different methods of formatting a comic script, but the essence remains the same: Page, Panel, Description, and Dialogue.
One of the biggest mistakes, I've found, is not writing enough description. The first panel of the page above could be written as, "A car is driving through a city," but unless you want the artist to just have free reign, this is not much of a description. Instead, you would be better writing, "An aerial view of a car driving down a city street. It is night, so its headlights are on. There are other cars parked along the side of the road." You could even add more specifics than that, if needed, such as the make of the car or the name of the street (if it should be based on a real one).
If you are writing your comic for submission to a publisher, chances are that you will only need the first 5 pages scripted to give to your artist, but if you feel so inclined, there is no reason not to write the entire issue.
I have written plenty of comic scripts, but it wasn't until recently that I actually found an artist for one. This, in my opinion, is the hardest part of the process. First of all, you need to find an artist whose style suits your comic. Every artist has a different style, so this is actually a lot harder than it sounds. There's no point in contacting an artist for your gritty, noir story if their style looks like Scott Pilgrim.
This also brings up another issue: contact. When you do find the perfect artist and throw them an email, you'll find that many don't do comics, or want to charge crazy page-rates, or simply don't reply at all. I think the only solution is perseverance. You need to keep looking, and keep asking.
To Be Continued...
So there you have my little summary of the initial stages of comic-writing. The next step, where I am currently residing, is playing the waiting game as your artist draws your pages. Once you have your sample pages, you can then begin the submission process. I've heard this too can be particularly gruelling, but again, I think perseverance is key.
Just as a matter of closure, here is a preview of pencil-work my artist has sent me for the page I sketched above: