Develop your idea first. Write a short premise that explains in a few sentences what your animated series is about. From the premise, develop the characters and write a detailed back story for your lead character and any important supporting characters. Animated characters today have lives beyond a simple cartoon story and can be as complex as any character in a non-animated production.
Develop a treatment that encompasses several proposed episodes for your series. The treatment should be written in third-person present tense and show the direction the story will be taking. It will also set the tone for the series. There isn't a set page number for treatments, but a treatment for an animated series might run between ten and fifty pages. The treatment will be part of any pitch you make to an animation studio.
Develop initial artwork for your project. It's not necessary that you be an artist, but consider working with one to get high-quality initial sketches of your main animated characters. You should also use a storyboard to help sell your animated series. A storyboard is a series of sketches done on a larger piece of cardboard in a comic strip fashion. The purpose of the storyboard is to demonstrate the overall look of the series characters in action. It's a good way to give your pitch audience (the producers) a look at the tone and feel of the animated series you're pitching. A storyboard is particularly important in a pitch meeting for an animated series, where the show itself relies on the animated characters as opposed to live actors.
Obtain an agent. An agent has contacts that can get you into a pitch meeting. Many animated production companies won't even consider a pitch if you don't have an agent. Consult an online database, such as afilmwriter.com, to find an agency. Contact the agency with a brief query that states who you are, what credits you have in the writing business, and a synopsis of the animated series for which you are seeking representation. You can sell an animated series without an agent, but the key is targeting studios that produce animation. A professional agent will know which companies are looking for animation and try to get you pitch meetings with those production companies.
Perfect your pitch. A pitch typically starts with a query letter, in which you will introduce yourself and write a brief animated series proposal that indicates some of the highlights of the series, such as the main character and the premise of the series. When your query letter is a success and you get invited to a pitch meeting, you will be expected to pitch your series orally and answer questions about the series to the executives in charge of buying and producing such shows. Practice your pitch extensively, as you should know your animated series inside and out; you want to be able to answer any question asked of you.