Just as you would outline a book, you can also outline a series.
Consider your series outline as listing the main objective you want to accomplish for each book. Just as each book will have an arc, your series will also have one. Your series outline should reflect the series arc.
Your series outline can be as simple as a bullet point for each book. That’s what I start with. Or you can add more substance to it, but I suggest you save additional details for the book outline, which you will need for each book in the series.
Outlining a series is fun, and I recommend it. Knowing a series arc, through its outline, can inform your writing of each book in the series.
For example, if you know your primary character for book three, make a subtle introduction in book one. This restrained reveal will delight your readers when they re-encounter that person two books later. Or if book eight has a plot development you worry may seem a bit contrived, with shrewd finesse lay the groundwork for it in books two, five, and seven. This unexpected development in book eight will still surprise your readers, but they won’t feel you forced it because you prepared them for it in earlier books.
Keep in mind that if you’re a discovery writer you can’t insert any delicious titbits into earlier books—unless they’re not yet published. But with a series outline to guide your writing, you can foreshadow what is to come in future books.
In addition, having a series outline will keep you from wasting time writing passages you will later cut. And your plan will help make your books richer because readers can connect with your writing and characters more fully.