Read as many animated screenplays as you can. The only sure-fire way to get the hang of script formatting is to learn from the professionals. Websites like Drew's Script-o-Rama (see Resources) have thousands of professional screenplays available to download, both animated and live action. Read as many as possible to see what a real script is supposed to look like.
Consider where your script is going to end up. An animated script written as a feature film is going to be paced and formatted differently than one that is intended to be a half-hour prime-time television episode. For example, an animated feature film can be much more developed and requires an action beat (a moment or scene where an important plot development occurs) every ten pages or so. On a half-hour animated show, because the running time is much shorter, you're going to need to adjust accordingly and include an action beat every two to three pages. Decide what your goal is before you start writing to save a lot of time.
Learn how to write to act breaks. In an animated television show you have to write in three act breaks (as opposed to the standard two for a live action show), or "cliffhangers" where commercial breaks will go. These are designed to entice the viewer into sticking around to watch the show when it comes back from the break. In an animated feature film you obviously don't have to plan for commercials so you have a lot more freedom with the act structure of your story. Your study of other scripts will help you.
Write emotion that doesn't rely on visual information. This is a tricky concept to master but is absolutely integral to the animated medium. Live action films can allow character development and moments of importance to play out silently with an emphasis on an actor's face. Since in animation you obviously cannot see the face of a voice actor, their vocal performance has to carry all the weight of their lines. Essentially, you have to write dialogue that would carry the same amount of emotion if you heard it with your eyes closed as opposed to actually seeing someone's face while he says it.
Write. You can prepare for a day or prepare for a year and the result will be the same. The only way you're going to write an animated script is if you actually start writing it. Lock yourself away in a room for hours on end. Turn off your cell phone and disconnect the Internet. Do whatever it takes to focus 100 percent on that blank page and get to work.