How do you make a custom map?

I'm working on my own game mode and don't really know how to start building a city/roleplay map?
Thanks in advance!

So most people use hammer editor, making a map isn't east but it's a good thing to learn how to do, and can be really good with a custom game mode i linked a beginners video below and that should teach you the basics from there. Good luck on your game mode and the map


https://www.youtube.com/watch?v=VNVBhTgqkBQ