Once your guild is created and counts at least 24 members (you need to be level 20 to create a guild), your Guild Master can book a city spot at level 40. Here is what you need to do:

The guild master needs to click on the the "Book a Spot" button under your Guild Village. It i located in the Guild Management Window, the third button from the left in the upper left-hand corner of the screen.

Pick a location from one of the three zones available to your guild. After you send the request, you will later a message saying that your spot is ready and you can teleport to your city spot and begin construction!