In the Beginning Was the Map: Mapmaking in Novel Plotting and TTRPG Adventure Design
Maps are very important to me. In both novel writing and roleplaying game design, I will commission a map extremely early in the process. It is a way of making the setting concrete in my own mind and in the reality of the imaginary world. Once the map is finalized, I will even constrain the story or the adventure to the confines of what the map dictates. For example, in my novel Nightborn, I had planned on one of the protagonists making her way from one location in a city to another. When I saw the map, I realized it was impossible for her to do that in the time required. The story changed, and the protagonist had to find another way to accomplish her goals.
I wasn't always this way.
For fifteen years, I was an editor in science fiction and fantasy publishing. Once upon a time, I had a writer from whom I'd acquired a fantasy trilogy. They emailed me to say, "I'll have the map for you soon." I was flabbergasted. "We'll commission your map," I said. "You don't have to pay for your map." But they insisted. I felt guilty, and they pressed, "I don't want to get something back and see that it's wrong and be told I have to live with it. So I'm commissioning my own map." I gave up protesting, but I still felt guilty.
Flash forward to when I sold the Thrones & Bones trilogy to Random House. And I suddenly realized, "They are a genius. I don't want my map not being anything but perfect. I had better commission it right away!"
For a while, I had admired the work of cartographer Rob Lazzaretti. Without knowing Rob, I called him up and engaged him in crafting my first map, a depiction of the land of Norrøngard from the novel Frostborn. While the novel was in edits, I was fortunate enough to be able to make a trip to Norway, the real world inspiration for my fictional land. I would take photographs during the day and email them to Rob at night. It was an amazing experience for both of us. The land that would shape my stories was being shaped by the land that inspired it!

A city map and a country map swiftly followed. When it came time to write the sequels Nightborn and Skyborn, I commissioned maps even earlier in the process. The shape of one strange peninsula even ended up being a clue in an ancient riddle in the second book.

By the time the trilogy was completed, I had more maps than had actually appeared in the books. Now that there are over a dozen tabletop roleplaying game offerings out or forthcoming, we've got more than twenty maps, quite a few of which have never been seen before.
These days I can open up a map and just stare at it and ideas start coming to me. It's like I'm walking down the streets of a real city, finding little cul-de-sacs and odd places I never noticed before. Asking "what goes on here?" or "why is this next to that?" leads me quite literally down streets I'd never venture on without the visual aid.
Next year, my company, Lazy Wolf Studios, will be launching Lands of Katernia, an exciting three-book project that features a monster tome, an adventure anthology, and a setting guide. It's the first project to pull the curtain back on all the other lands on the continent, places that have appeared in novels so far but not in games, and even more places that have never appeared anywhere. Rob Lazzaretti and I are already working hard on creating and illustrating new cities, from a city recently liberated from enemy occupation to a city stretched across multiple small islands where humans and naiads co-exist. I'm so excited to show it to you.

Of course, for your own writing, you don't have to do as I did. There are several good mapmaking programs out there that can at least get you started, from complex programs like Campaign Cartographer to easier ones like Inkarnate, though I will always champion working with actual humans over anything else. That's where the creative synergy and happy accidents lead you to those aforementioned little cul-de-sacs. And there's nothing wrong with taking up pen and paper yourself. If anything, your efforts can't be as bad as my very first map of my world.

However you approach it, working out the where can be a good method for inspiring the what, the how, and the why. Mapmaking is just one tool when it comes to creating narrative, but it is one of my favorites.
If you would rather listen and watch than read, here it is.