It seems to me that Area 51 should tell how to propose a new site. However, this is what it says to do to create a site:
How do I start a new site?
If your area of expertise doesn't already have a Stack Exchange site, propose it! Stack Exchange sites are free to create and free to use. All we ask is that you have an enthusiastic, committed group of expert users who check in regularly, asking and answering questions.
The creation of Stack Exchange sites is a democratic, community-driven process. That does not mean the majority of Area 51 users have to love your site idea. It simply means you have to recruit a community of users large enough so that questions get good answers quickly. Reach out to other experts to build support for your site; bloggers, enthusiasts, and support groups can all benefit from a world-class, canonical collection of expert answers to the hardest questions.
Note that it mentions finding a group of experts (Which is important), but doesn't say anything about the mechanics of proposing something. It seems that this is a bit of a mis-hap. I do know that it says on the bottom of the main site, as listed below, but it still seems like that should be in the FAQ...
Can't find what you're looking for? Select a category listing to the left and propose your site there!