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Some genres, such as fantasy, science fiction, and mystery, are more amenable to a series than others, like romance or commercial fiction. Decide whether your concept fits into the former or the latter.  Children’s and young adult fiction are other genres that are suitable for series building. Some publishers prefer fantasy series over individual fantasy novels, because fantasy series tend to sell better than standalone fantasy books. You will have to decide whether you are interested in writing one large story broken into several books, such as the Harry Potter series or a group of stand-alone books featuring the same characters, such as Agatha Christie’s Hercule Poirot mysteries. Making this decision will help you figure out whether you will plot each book separately, or whether you will try to have an overarching storyline between the books.  Did you just finish a novel and don’t want to say goodbye your main characters? Or, have you developed a fantastic plot that is so wide-ranging it needs to be told over several installments? The former motivation could lead to a long-running series, like a group of mystery novels, while the latter could lead to a trilogy.  You will also need to figure out whether you will have an ensemble cast that appears throughout the series, or whether you will primarily have a main character who meets new characters in each book. To help you plan out the series, create a writing timeline with deadlines and due dates. Determine how much time per week or per day you are going to devote to writing the book series. Have deadlines for completed chapters or for a certain page count so you are encouraged to complete the book series.  For example, you may have a goal page count per week that you have to hit. Or you may have a certain number of chapters you have to complete every month so you can get the book series done. You will need to be prepared to write and publish frequently as a series writer. Your audience will want to know what happens next to your characters. You also don’t want your readers to move on to other books or series if they get tired of waiting for your next installment.
Choose a genre for the series. Consider the type of series you will write. Make a writing timeline.