Choosing to write a series is an important decision. Usually, series are born from stories that are too big for one book. However, a series can also be built from extensive planning of individual stories for each book.
Either way, a series usually follows a character through different stages of their life and various adventures. The characters evolve with each journey because the books build onto the story with each new book.
It’s important to make sure you have enough story, not fluff, so that people will continue to read through the series. Have you ever noticed that sometimes the sequel to an amazing book or movie sucks? It’s because there isn’t enough story left to continue the series properly. The sequels should be a new story with the same characters which transitions smoothly from the last book.
If you do have enough story for a series, planning is crucial. You have to know your characters inside and out, their birthdays and habits and hair color. You also have to know your world. Is it based on reality or a world you created? Either way you need to know everything about it. Then, plan out your timeline. Timing is essential because you have to know when to include each plot point and grow the characters with each conflict.
After you have all the information you need, you can either freewrite the story or outline your story and write from there. Keep going until your satisfied but know when to call it quits. A series is only as great as it’s characters.