Dang! Casual Notice responded while I was writing up a reply - so I'm just going to go ahead and post mine anyway, even though there is some overlap with his guide!
Firstly, welcome to the forums.
As a bare minimum your script must include every detail that's important for the story. For each scene you should specify:
- Characters present
- Props, clothing or objects
It is important that these are specified before the artist begins work otherwise they may draw an entire scene only to discover at the end a line that says "Jeff, watching from the side, puts up his umbrella and walks though the archway". If the artist wasn't aware Jeff, his umbrella or the archway were going to appear, they will have a lot of editing to do, to incorporate these features.
Elements such as the panel layout, action sequences, position of characters, lighting, etc. may
be important to a particular scene. Again, where important, specify it.
Other than that, I'd imagine it can vary from team to team. If you have a very specific view of what a character, scene or prop needs to look like, you need to specify it in detail or it will not look as you expected. If you are happy to let the artist have free-reign over these matters, you don't have
to specify them at all
and can enjoy the surprise! As a general rule of thumb you might find if you are not paying an artist, giving them more flexibility will keep them interested longer (although of course all artists are different and some may prefer a detailed brief).
you enter into an arrangement with an artist, you should talk about your expectations, particularly to do with money. You may want to sign a contract to be clear about ownership and the distribution of profits. You also need to make sure you agree realistic timescales.
Make it clear if you are presenting the artist with a draft or a final script. If it is a final script as a minimum, it should have been checked for spelling and grammar.
If you are new to writing scripts: avoid large chunks of text, show what's going on rather than having the characters describe it and be aware that the work involved in scripting can be very different to drawing - even small action sequences may need multiple panels to illustrate (ie "The window opens slowy" is four words, but may take several drawn panels to demonstrate the length of time the window is taking to open).