First, think about the personality of Baby Bowser Jr. Is he mischievous? Naive? Greedy? Then, come up with a basic plot. It could be as simple as him wanting to steal a special power - up from Mario. Next, create some supporting characters. Maybe some loyal Koopa minions or a new, made - up character. For example, a baby dragon who becomes his sidekick. Then just start writing, describe the setting, like the dark and spooky Bowser's castle or the bright and colorful Mushroom Kingdom.
To start writing Baby Bowser Jr. fanfiction, you should do some research on the existing Mario universe. Look at how Bowser Jr. is portrayed in the games. Then, you can begin with an interesting hook. For instance, start with Baby Bowser Jr. in the middle of a prank gone wrong. After that, develop his character arc. Maybe he starts off as a bratty little villain but through the story, he learns some important values. Don't forget to add some action and adventure. He could be on a quest for a powerful artifact that will make him the most feared baby in the Mushroom World.
Begin by deciding on the theme of your fanfiction. If it's about friendship, you could write a story where Baby Bowser Jr. makes an unlikely friend. If it's about self - discovery, it could be about him finding out his true potential. Once you have the theme, think of the conflict. Will it be external, like fighting against Mario? Or internal, like dealing with his own insecurities? Then, write a detailed outline of the story, including the beginning, middle, and end. Start with a strong opening scene that grabs the reader's attention, like Baby Bowser Jr. plotting his biggest scheme yet.