This is where things can get compicated if you want, just like fried chicken or chili there are many ways to to it, and just like those the way you do it is best.
Some used milks and flour, some use water and flour some add sugar, I do it simple, I just feed it flour and water.
What you need to do is induce a yeast into it, an active yeast. The bacteria is in the air and sometimes you can get a good yeast from the air, some areas like the San Francisco area or the Nome area are famous for good wild yeast. My friend Capt. Hamp Cox likes yeast found on wild grapes. I found I like to buy my yeast, but regular bread yeast don't work well, it tends to get weak very quickly. I like to make yeast type sourdough and I don't want to add yeast to the dough as some do.
What I have found is the best and this is based on some help from a couple of micro-bioligists, I use a yeast called Saccharomyces bayanus and I buy a package for a $1, if you are a yeast expert you already know this is nothing but French Champagne yeast. It has a high tolerance to it's own alcohol and it of course is famous for making CO2.
I simply dissolve it in about a cup of warm water in a 1/2 gallon jar, I add another pint of water and stir in a couple of cups of flour, I add more each day and use more flour than water till after about 3 days I have a jar that is almost full of something about like thin pancake batter. By then it has that sour smell and is ready to use.
I try to use it at least 2 times a week and replace what I use, usually about 3/4 of it.