How do you make bread from literally flour and water?

I am always buying bread and I figure I may as well make it myself. I want to try making it strictly from flour and water. My understanding is that you can mix the two and basically leave it for a day or two, then bake it into nice and tasty bread. I recognize that this may not be the most delicious bread, but it's most certainly the cheapest and I'd like to try it. I have searched online and almost all recipes just say to buy yeast. Obviously bread was made before you could buy yeast at a grocery store in a packet.

How the hell do you make a loaf of bread?