If you don't want to use a commercial sour culture for this, you can do the following, which I have tried and it worked *very* well:
Start with some weak wort around 4*P (1.016). A few tablespoons of DME in a liter of hot water will do the trick. Toss in a handful of grain. Stick it in your oven at 120*F overnight if it has a warm setting like mine; otherwise you can use a heating pad and play with it until the thermometer reads around 120. Whatever you do make sure it stays over 110*F. You don't want the bacteria to grow that make poopy diaper flavors. *yuck*
The next day, make up a new batch of weak wort but keep the volume at 750ml this time. Toss in a tablespoon of dried milk, or use some milk instead of water. The lactic acid producing bacteria love milk sugars. Pour 250ml of the first starter into this one and toss the rest of it. Stick this second starter in the oven or on your heating pad again and let it work for a full 24 hours.
Now, the important part -- give it a taste. It should be tangy, but clean -- with a cheese-like flavor and aroma. If it's funky, repeat the build-up step to let the lactic bacteria outcompete the funk. If it tastes and smells good you're ready to use it.