Writing a fantasy story is based on the same principles I presented in the ten episodes of "Learning to Write." What else should you pay attention to if you want to write a fantasy story? I think you only start if you're a true fantasy reader. You're captivated by the genre and enjoy fantasy films, which are often series. You're undoubtedly familiar with "Game of Thrones," to name just one. You read the fantasy books of Terry Brooks, who I consider one of the best fantasy writers. Here too, I'd like to focus on one author, but there are many, many more whose books are worth reading. Incidentally, a fantasy story often spans three books, a trilogy, or even more. In that case, it's called an epic, like "The Wheel of Time."
You've definitely decided to write a trilogy and have done the necessary preparation. First and foremost, you have to come up with something original, and that's not easy. What's still original, something that other fantasy writers haven't used in their stories? There really isn't one. In pure fantasy stories, magic, gods and goddesses, magical animals, and magically possessed people almost always play a role. Furthermore, it's a battle between good and evil. Keep in mind that evil is defeated in the final chapter. That could be the central theme of your long story. You could also hint, for example, in the (short) epilogue, that there might be a sequel, but keep that vague.
Fantasy is the creation of new lands with people who differ in appearance, culture, language, education, etc. Start by drawing a map with countries, seas, rivers, forests, cities, and whatever you consider truly important. But don't be too detailed; limit yourself to the main points. It's a different story if all the action in the story takes place in a single city. In that case, you need to draw a reasonably detailed map of the city.
You have the story mostly in your head, and you're already creating the main characters for yourself. Think about their names. You can find many names that will appear in your story on the website https://www.fantasynamegenerator.com/ It's very comprehensive and can help you with names for people, animals, countries, etc. Make sure you have the names of the characters in your story, including their appearance, their abilities, such as their magic, and any other special characteristics, at hand.
It's time to put the first words on paper.
More next time.
Rolf