“... the Code is more what you'd call guidelines than actual rules.” — Captain Hector Barbossa, Pirates of the Caribbean: The Curse of the Black Pearl
This is kind of a grab-bag of both storytelling suggestions and practical suggestions, aimed a writer who is going to be working with an artist who is not them. Of course, like every set of "rules" in art, you can break them if the result is better than following them, but it's a good idea to know what paths have been trod on to avoid obvious bad directions.
The essential grammar of sequential storytelling has been around for nearly 200 years (and, of course, people were telling stories in art for millenia before that). It's a good idea to know that grammar, because comics readers are highly acclimated to it, and they don't have to work as hard if you are following it. (Of course, break it if it works better.) Understanding Comics is the first place to start to learn about how to tell stories with pictures, but there's a universe of stuff out there.
You can tell a story with pictures in a thousand ways, of course, and if you want to do something other than traditional comics style, go for it! The best teller of illustrated stories alive today (IMHO) is Chris Ware, and his work is nothing like a Marvel comic.
For a writer who doesn't draw, comics are inherently a team effort. It's OK to put in camera angles and things if they are important, but it is ultimately the job of the artist to make the story work on the page. Be receptive to their ideas. Don't over-specify the art.
Most male artists who you run into who grew up wanting to be comics artists grew up wanting to draw Batman. They tilt towards the superhero look for both settings and characters. If you are writing a comic where the people are normal, you may have to gently work with them to get the characters to not look like Bane.
If you know who is going to be drawing the comic, write for their art style. If they don't do detailed backgrounds, for example, don't make the plot depend on background details.
There is no standard script format for comics, but Microsoft Word .docx is the lingua franca, followed closely by PDF. It is conventional to start a new script page for each comic page. I don't like movie screenplay format for comics, because movie screenplay format was designed with some very specific goals in mind that don't apply to comics, and it takes up a lot of space. But just make sure everything is clear, and if the artist wants something different, give it to them.
Comics are second only to movies as a show-don't-tell medium. Dialogue adds detail, but should not be the spine of the story.
Comics aren't movies. This is the single biggest problem I see in a new writer's scripts. The pictures don't move. When writing descriptions, remember that it will be fixed on the page. Don't write motion, but stills:
No: Panel 1: Joe walks across the room, and sits down.
Yes: Panel 1: We see Joe sitting down in the chair, half-way there.
No: Panel 1: Joe flinches, then recovers.
Yes: Panel 1: Joe's face is in a flinch. Panel 2: Joe's expression recovers.
Never ever: Panel 1: Joe runs across the cornfield, jumps into his car, and drives off, his taillights disappearing in the dust the car kicks up.
A good rule of thumb is that if a panel description has more than one simple action in it, it's going to be hard to draw accurately.
When thinking about what your characters look like, keep them as physically distinctive as possible. (Unless, of course, they're identical twins or Doombots or something.) You should be able to tell just from a silhouette who is who. This isn't as easy in reality-based comics as it is in superhero comics, but hair style goes a long way.
Pages are not as big as you think they are. More than six panels per page pretty much means "no backgrounds" and "only one character per panel." Do not do more than seven panels per page, and save that for special occassions. (Yes, Alan Moore often writes more than seven panels per page. You are not Alan Moore.) I find that five panels per page is the sweet spot, because it allows an A / B / A / B / C rhythm. (And, to be totally honest, it pushes the artist to come up with a page design that is not grid after grid.)
Dialogue! This is an area where most new comic writers fall down, by putting way too much dialogue in the comic.
Keep panel descriptions as short as possible in order to get the point of the panel across to the artist. If you want to give lots of complex backstory, put it at the start of the script, or in an appendix. Going into a lot of psychological depth for a character can help some artists (but not all) in how they design the character, but don't make the artist wade through three paragraphs of worldbuidling to get to "He walks out the door."
A reference image is worth at least a thousand words of script trying to get the point across to the artist.
The pages are the major heatbeats of your story. Use page breaks wisely.
Supplemental works: