Well, start by choosing a specific aspect of the 'How to Train Your Dragon' universe to focus on. Maybe it's the relationship between different dragon species. Then, think about the plot. It could be a mystery where a rare dragon goes missing and the characters have to find it. When it comes to characters, you can introduce new ones but make sure they fit in the world. For instance, a foreign Viking who has a unique way of training dragons. As for the writing style, use descriptive language to bring the dragons and the action to life. Describe how the dragons look, how they fly, and the bond between the riders and their dragons.
First, know the world of 'How to Train Your Dragon' well. Re - watch the movies or read the books to get all the details about the dragons, the characters like Hiccup and Astrid, and the Viking village. Then, come up with an interesting concept. It could be a new adventure for Hiccup and Toothless, or a story about a different dragon that hasn't been explored much. Next, create your own characters or expand on the existing ones. For example, you could give a side character more depth or create a new dragon rider. Start writing, keeping in mind the tone of the original - it's usually a mix of adventure, friendship, and a bit of humor.