First, do your research on both Harry Potter elements and velociraptors. Know the rules of magic in the Harry Potter world and the behavior of velociraptors. Then, come up with an interesting plot. It could start with a mystery, like why are velociraptors suddenly appearing in Hogwarts? Next, develop your characters. How will Harry and his friends react to these strange creatures? And finally, write in a way that captures the essence of both the Harry Potter series and the uniqueness of velociraptors.
To write this kind of fanfiction, think about how the two concepts can blend. You could start by imagining a world where the Ministry of Magic has a secret project involving velociraptors. Maybe they are trying to use the raptors' hunting skills to track down dark wizards. Harry and his friends get involved when they stumble upon this project. As for the writing style, use vivid descriptions to bring both the magical and prehistoric elements to life. For example, describe the look in a velociraptor's eyes as it chases after a dark wizard through the dark alleys of Diagon Alley.