Preproduction is my favorite part of working on animation. I've read a couple of books on the topic but in my experience, nothing beats
just doing it. My personal writing/storyboard experience is mainly in shorts and commercials but the basic principles are the same for longer format productions. Here are some basic tips:
I like to start by writing a treatment, which is basically breaking down the story to its main 'story points'. This should be kept very short--as little as a single sentence, no more than a brief paragraph for each act (if it's a long piece) or sequence (if it's short.) Think broad strokes. This stage is very important since this is the structure you will follow when writing the script.
Writing the script is essentially filling in the details of the treatment--keep it manageable by tackling each story point separately. If you get stuck, move onto the next story point and work on that. There is no reason to write in a linear path. Sometimes it helps to write the ending first or second, then it's a matter of connecting the points to reach that conclusion.
If the characters haven't been previously established, it may help to write a brief history for each character to help you guide their actions and dialog.
Here's a very important tip: write very quickly, and don't fuss about editing or making things perfect. You can always come back for that later. The important thing is to get a first draft completed while you're feeling fresh and excited about the subject. If you stop to edit and fuss during the first draft, the project will likely lose momentum, and you'll eventually become frustrated with it.
I try to write with visuals in mind, but save most of the full description for the storyboards. As they say, a picture is worth a thousand blah-blahs. Keep the visuals and action within what can actually be accomplished in the available time, budget and technical capabilities.
Designing your production fits in here somewhere. I might start designing while I"m doing the storyboards, but before that I try to get a
first pass of the boards done with as little distraction as possible. Thumbnails are all you need for a first pass (remember 'broad strokes.') Just like with the above stages, it's important to get that first pass out as quickly as possible and not worry about making it pretty during the process. Once you're done with your first pass, you can come back and work on editing and improving it. At that stage, you can choose to make it pretty if you wish but keep in mind that
clarity is important if you're going to work with a team. If you give a team good clear instructions, they can work on making it pretty. (Of course if you're also working to please a client, making the boards look 'pretty' probably should take greater priority.)
If you're looking for technical information about screen writing, like proper formatting and terminology, all that info can be found online. Just do a Google search for screenwriting reference, format, standards, etc. Also do a search for specific screenplays--there are many websites that have TV and movie scripts you can download and study, including animated productions.
Hope this helps.
G.