To start a Pokemon Monster Hunter fanfiction, begin by brainstorming ideas for the plot. You could look at the unique features of both universes and find ways to combine them. For instance, the way Pokemon evolve could be related to the way Monster Hunter monsters grow or change. Once you have a general idea of the plot, create your characters. They could be a combination of Pokemon trainers and Monster Hunter hunters. After that, set the mood and the world - building. Describe the environment, whether it's a forest full of both Pokemon and Monster Hunter beasts or a city that has elements of both worlds. Then start writing the first chapter, introducing your characters and the initial situation.
Start by choosing a theme that ties Pokemon and Monster Hunter together. It could be about friendship, like a Pokemon and a Monster Hunter's companion forming an unlikely bond. Then, do some research on both franchises to get a better understanding of their lore, characters, and mechanics. After that, create an outline for your story. Decide on the beginning, middle, and end. For the beginning, you might introduce a new character who is interested in both Pokemon and Monster Hunter. In the middle, they face challenges that involve both Pokemon and Monster Hunter elements, like a powerful new Pokemon - like monster that they have to defeat. For the end, think about how the character has grown and what they have learned from their experiences in this unique crossover.