In complete contrast to the ‘Reality’ world type, a fantasy world type, is created entirely from scratch by the author. The fantasy and science fiction genres do this all the time. Famous examples include Middle Earth, Discworld and the Star Wars Galaxy.
The real trick is with building a fantasy world is having a very good understanding of what you need from your fantasy world in order for the story to function. You need to have a clear idea of the main settings that your characters will be interacting with, and then start to build the picture up from there.
For example Tolkien starts out in ‘The Hobbit’ with a very simple setting. He describes Bag-End. He then slowly moves the story throughout Middle Earth describing Elrond’s House, Gollum’s Cave, Mirkwood, Lake Town and the Lonely Mountain. The settings used in ‘The Hobbit’ are comparatively small compared the the more extended world of ‘Lord of the Ring’s’ which takes in the Mines of Moira, Lothlorien, Rohan, Gondor and Mordor to name just a few. Those aren’t just the only places you see on the map though; Middle Earth is a lot bigger than that. The place called Middle Earth is still only part of Tolkien’s world as well. The Undying Lands in the West are also prominently featured in his works and in the consciousness of some of his Middle Earth characters.
Tolkien is a great example of world building; his world is quite small though compared to the entire galaxies of diverse civilisations and worlds that you see in creations like Star Wars and Star Trek. The infinite space of the universe does make for the possibility of greater extensions of the imagination. It is this infinite possibility that can make building your fantasy world from scratch incredibly daunting, especially when you start to consider all of the little details that you might need to know such as the religions of the world, the names of all the different countries and regions, how justice works, where food comes from, how the different and diverse peoples of your world interact with each other.
If you have confidence with your imagination great, use it. However, like I recommended in my post about building a world based on reality, using the world we live in as inspiration is just as limitless as creating an entire galaxy from scratch. Use history and our world as inspiration. It can be as small as being inspired to transform Hadrian’s Wall into The Wall, like George R.R. Martin does in Game of Thrones, or it can be a more intricately complicated use of an existing culture. Lian Hearn’s trilogy ‘The Tales of the Otori’, uses the culture of Feudal Japan as an inspiration for the type of world the characters live in.
What you need from your fantasy world though only you can truly know; all I can suggest is start small and expand from what you need for the story to what you need just to add a little bit more depth. A lot of the world-building series will be looking at the extra details you can consider when building a new world, but for now just remember don’t panic, don’t lose yourself in building the world so much you forget your story, and don’t forget while you have your imagination we do have limitless amounts of inspiration from our world as well that you can use as a part of your building blocks.