To start a Kid Buu crossover fanfiction, begin by brainstorming the crossovers that would make the most sense or be the most exciting. Let's say you choose the Star Wars universe. You should establish the rules of the crossover early on. How does Kid Buu's energy interact with the Force? Then, create a hook. It could be something like Kid Buu being drawn to the dark side of the Force. Now, introduce the Star Wars characters. Luke Skywalker might sense Kid Buu's presence and be determined to stop him. You can then start developing the plot by thinking about what Kid Buu wants in this new universe and how the Star Wars characters will oppose him.
Well, when starting a Kid Buu crossover fanfiction, you have to pick the right crossover partner. If it's the Transformers universe, think about how Kid Buu would interact with the Autobots and Decepticons. Once you've decided, start with a bang. For example, Kid Buu could crash - land on Cybertron. Then, describe the chaos that follows. The Transformers would be both curious and alarmed by this strange being. Now, start bringing in the main characters from both sides. Optimus Prime might try to communicate with Kid Buu, while Kid Buu just wants to destroy. This creates an immediate conflict and sets the stage for the rest of the fanfiction.