1) Get a substance that can act as food for the little beasties.
2) Let the little beasties eat, and produce something that wasn't there before.
This guy in the picture is my sour sponge. I've pulled him out of the fridge, and I'll let him sit on the counter and ferment for a few hours. Sometimes I'll let him go over night, but I'm running a little behind today. Winter in Utah isn't great for this project because it's cold and dry, and this guy prefers warmer and humid. With luck that will be changing for us this year, but that's a subject for another day.
After he's fermented for a few hours I'll do a couple of things.
First, I'll remove a chunk about the size of a small orange, and set that aside for a sour starter that will ferment further over night. I'll make the starter by adding 2 cups of warm water to the chunk of sponge, and mix that until it's all blended together. I'll cover that, and let it bubble away...and bubble it will.
Second, I'll put the rest of the sponge in my mixer, add a cup of flour, and some water, and mix that for about 4 minutes until it becomes the consistency of stiff dough, and I'll put that back in the fridge to wait until the next time I need to do a little baking.
Tomorrow morning, using a modified version of the recipe found here: http://whatscookingamerica.net/Bread/SourdoughPancakes.htm
I'll make what my 5 year old considers to be the best pancakes in the world.
No, it's not brewing, but essentially it's the same thing. It's the process of taking a product derived from grain, creating the ideal, or as ideal as possible, of an environment for yeasts to thrive, and allowing nature to take its course. In the end, all we do is try to create the ideal environment, but ultimately, this process will do as it sees fit, and all we can do is sit back and hope for the best.