First, you need to understand Ginny's character well. Think about her personality traits, her relationships, and her goals. Then, decide on the plot. For example, is it a story about her unexpected pregnancy? Or is it a planned one? Next, create some conflict. Maybe she has to deal with someone who doesn't support her pregnancy. Outline the story from the start of her finding out she's pregnant to the end, whether it's the birth or some other significant event. And don't forget to add details about how she feels physically and emotionally throughout the pregnancy.
To write a Ginny pregnant fanfiction, start by choosing a unique angle. You could base it on a particular time in the original story or create an alternate universe. Develop Ginny's character further in the context of pregnancy. How does she handle the news? What are her fears and hopes? Incorporate other characters as well. Maybe her friends have different reactions. Write about the different stages of pregnancy realistically. Describe the doctor visits, the cravings, and the mood swings. Also, give the story a satisfying ending, whether it's a happy family moment or a bittersweet realization.