First, you need to have a clear idea of the characters you want to involve. Decide if you'll use the main Harry Potter characters or create some new ones. Then, think about how rap will be incorporated. Will it be a secret talent of a character? Or a new form of magic? For the romance part, plan out the relationship arc - how they meet, their first impressions, and how they fall in love. Start with an interesting opening scene, like a rap battle in the Hogwarts courtyard.
To start writing your Harry Potter rap romance fanfiction, begin by immersing yourself in the Harry Potter world again. Re - read the books or re - watch the movies to get the feel of the setting, the language, and the characters. Next, consider the role of rap. You could make it so that rap is a way to break spells or a form of magical communication. For the romance, think about the chemistry between the characters. What makes them attracted to each other? It could be their shared love of rap or a common goal within the wizarding world. Write a detailed outline first, including all the major plot points, and then start filling in the details.