Real sour milk is simply fresh milk (unpasteurized) that has been left at room temperature until sour. It will happen overnight. I don't know how authentic you're trying to be, or even if you have access to raw milk, but you can't do this with pasteurized milk, which has been treated to kill ALL the organisms, both potentially harmful and beneficial, so it won't sour, it will just go putrid. If you're using pasteurized milk, your best bet is the vinegar or lemon juice method Amanda explained. It is sour, though not in the same way. I personally can't abide the taste of soured milk, since it's not really a thing I grew up with, plus, you really have to plan ahead!
I'm not certain what it is supposed to be, but my "Red Devils Food Cake" recipe of my Grandmother's calls for it. It suggests, "1 cup whole milk with 1Tbs white vinegar" to create it. I let it sit about 2 minutes after stirring before adding it.
There are a couple of different ways to make it, and here's the way I do it: mix 1 cup milk with 1 teaspoon vinegar. Stir. It will begin to look a little lumpy. That's it!
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I let it sit about 2 minutes after stirring before adding it.