Determine whether your storyline is a good match for animation. If it involves lots of action, sound effects, anthropomorphic characters (animals or inanimate objects that act like humans), or fantasy elements, animation will probably work. If your plot has complex layers of subtext, introspective characters, lots of long conversations, or character reactions that are primarily visual versus verbal, it's not a good candidate for animation.
Identify your target audience for this film. Although people commonly think of animation as being kid stuff, there are lots of sophisticated scripts written for adults that contain political satire, double meanings and pop culture references. The target demographic also dictates the length and complexity of the film. Specifically, the younger your viewers, the shorter and simpler your story.
Make a list of the characters in your story and how many settings are needed to carry the plot. Identify who the protagonist is, what this protagonist's goal is in the story and who or what opposes him in reaching his goal.
Create a working outline before you start writing. The outline needs to contain a beginning, middle and ending. If you're new to screenwriting and not sure how much each section should contain, the easiest formula is to divide your desired number of pages by three. Example: if you're writing a 30-minute animated film, each act will be 10 pages long since one page of text equals one minute of playing time onscreen.
Make each line of dialogue and every conversation count toward: revealing character; disclosing information; raising the stakes; and/or resolving the conflict. Keep your animated characters' lines of dialogue short in order to maintain a snappy pace.
Use lots of action in your animated screenplay and describe in detail what the characters are doing. Example: An exhausted Joe reaches the top of the mountain. Everything starts to shake. The mountain is a volcano that erupts in red, orange and gold flames. Joe loses his balance and tumbles all the way down. He lands on his lunchbox and crushes it.