When you start out as a writer, you are usually advised to write what you know. This is pretty good advice. It cuts down on the need to do masses of research, and it can make your writing more grounded in concrete detail, and less vague and waffly. And the things you know a lot about are often the things closest to your heart. Writing that comes from the heart will generally pack more punch.
But if you're meant to write what you know, how can anyone write fantasy fiction? How can I know about Laridy, a city that's completely made up? The answer is that I do know it, in a way. I've spent hours and weeks and months living in that made up world, until I know its people and its ways.
I can also take the stuff I know in the real world, and build that into my fantasy realm. This imported knowledge (so long as I adapt it to fit in with the strange ways and history of Laridy) will have that ring of authenticity to it, because I know what I'm talking about. An example here is the martial arts in Wolf Tide. I've been doing judo for about twelve years now, and I've also done some karate. In my novel a version of karate (with spinning back kicks) is what the Galen race go in for. The people from Bogganburg have a different martial art, Boggan wrestling, and this equates more to judo.
My flying detective, Anabara Nolio, is a fighter. I'm writing her character from what I know. And the stuff I don't know directly, I spend a long time imagining, so that in the end, it's as if I know it.
This is my book about how I got my judo black belt. I'd love to tell you I also have a black belt in karate, but that would be pure fantasy.