Begin by imagining a world for your Pokegirls fanfic. Decide on the rules of this world, such as how Pokegirls are caught, trained, and interact with humans. Next, develop your characters' personalities. Are they brave, shy, or mischievous? Once you have these elements in place, start writing the opening scene. It could be an exciting encounter, like the trainer stumbling upon a wild Pokegirl in an unexpected place. As you write, let your creativity flow and don't be afraid to deviate from the traditional Pokemon concepts.
First, you need to come up with an interesting concept. It could be a new type of Pokegirl, a unique adventure setting, or a different take on the relationship between trainers and Pokegirls. Then, create some basic characters, like the main trainer and the Pokegirls they will interact with.
To start writing a Pokegirls fanfic, think about the plot. You could have a mystery - like a missing Pokegirl that the trainer has to find. Or a competition - where the trainer enters their Pokegirl in a special tournament. Outline the main events that will happen in your story. Also, do some research on the existing Pokegirls in the official Pokemon universe to get inspiration for your own creations.