Write an outline. Determine how long the episode or animated movie will be. Write an outline that details the major high and low points in the story. Develop story points or "beats" that are crucial to the story and action. Develop the story around these "beats."
Write about the characters. Many screenplay writers develop their characters by using a technique called back story. Back story details what the characters are about and helps to provide motivation for how and why characters act and react. Character back story helps the writer learn reasons why some characters react in certain ways; knowing this helps the writer to move the story forward.
Use the outline to determine action sequences. One of the major ways an animation script differs from a live-action script is through the use of action. An animation script contains lots of action. Animated characters can be involved in many more perilous situations than live-action actors. Action sequences should be written very clearly and concisely. The animator uses the script as a blueprint to create the action sequences.
Write dialogue. Dialogue, as in live action scripts, helps to move the story forward. Live-action scripts can proceed at a slower pace than animation scripts as live actors can interpret their parts as they interpret their character. An animation script moves quickly; it is the nature of animation to keep things, characters and situations in motion. Animation characters generally speak in short, quick sentences. Character dialogue should be sharp, quick and snappy. Long speeches in animation are boring.
Rewrite. After the first draft is completed, let it rest. Review the script for movement, action and dialogue. Rewrite any action sequences that are not clear or concise. Shorten dialogue as needed. Make sure the format is correct. Check for any typos.