The next step in my very professional, evidence-based, and definitely not at all designed by me and me alone process was to play some games. This step was pretty much nonexistent in the process for my previous books for a couple of reasons. First, they required less world-building because they took place in real locations. And second, I started them during times when I didn't have as much free time as I do right now. But this time, I need to do some world-building and I have some extra time, so games it is!
My go to game is pretty much any dress-up games on the internet. I discovered the value of these because I remembered once how I played them when I was younger and thought, "Hey, that would probably be rather useful for character designs!" (Except not really that coherently). So I'll use whatever dress-up character creator thing happens to match what I need for my character and use that to visualize it. This has proved to be really good at getting me in the mindset for writing, as well being a very helpful guide for drawings in my big book of characters (there's a blog post about that here).
The other game I've been using for this step is Minecraft. My brother and I discovered Minecraft years ago as a fun game to play together. It's also a great tool for world-building. Not only is it good for visualizing the layout of things, but also for how buildings look on the inside. I will say that it may not be the best tool for a situation where you need a whole world, but this book takes place in one city, so it works just fine for me.
Takeaways from this step: Well... basically that games are totally helpful to me as an author and nothing to be embarrassed about.