Start by choosing the right crossover for your Rabbids Fanfiction. If it's a crossover with the Kirby series, think about the similarities and differences between the two. The Rabbids are known for their craziness and Kirby for his cuteness and power - up abilities. Plan out the basic structure of your story, like introducing the Rabbids in Kirby's world. Then start writing the first chapter, focusing on creating an engaging opening that will draw readers in.