There are many ways to script a comic but we’re going to discuss the style I picked up when first starting out in comics. It’s a modified version of the style Neil Gaiman uses, which is apparently a modified version of the style Alan Moore uses so you know it’s a scripting style that works.
A comic script is laid out in two main sections: description and dialogue. You start with a line explaining which page and which panel you are discussing, followed by the description on the next line down. If you have a particular panel layout in mind, you can start with a line saying Page One (or whatever page it is) followed by a quick description of the preferred panel layout on the next line. Then you’d go to the page X, panel Y line and, after that, the panel description.
When scripting a comic it’s important to not overwrite the description of what you want shown in each panel. The exact amount of detail you send to your artist varies based on you and your artist’s level of understanding of the story and the characters you’re using. So with an artist that’s new to the comic you might prefer to add more information about who everyone is, and the way they usually act in the given circumstances.
For example, if you have a gruff war hero character like Captain Jawbone, veteran of the 5th interstellar war, you may describe to a new artist his tendency to show no emotion around people he does not know well. This ensures the artist won’t think the Captain would smile a lot, or be seen laughing at a stranger’s jokes.
You could leave that out with an artist who has been drawing the character for years, because it’s fair to say they already know this stuff. For a veteran artist it’s usually enough to say what each character is doing in a particular frame, and let them decide how best to draw the scene based on that.
Beneath the panel description is the dialogue section. This is laid out in a similar style to a film script: dialogue is centred, to make it instantly clear that we’re not looking at description any more. That is especially important for your letterer, as they can spot which parts of the script they need to concentrate on.
The first line of your dialogue should be the name of the character who is speaking. If you’re writing a caption rather than a character, simply write “Caption” here instead of a name. Drop to a new line and begin writing your dialogue, or the contents of your caption.
All dialogue and caption text should be written in All Caps. Again, this differentiates the dialogue section from the description. If any part of the dialogue should be in bold or italics, make sure it’s emboldened or italicised in your script.
Continue with your dialogue in this same format until everything is said that needs to be said. Then rinse and repeat with the other panels and other pages, until your script is complete. That’s really all there is to it.