Forget fancy starter recipes—especially ones telling you to add baker’s yeast! Just put a little whole wheat flour in a small dish and mix in some water till it’s like pancake batter. Then cover it and set in a warm place, but out of direct sunlight.
Exact amounts really don’t matter, but if you need a guideline, try half a cup of flour with an equal amount of water. After evaporation, that should yield about half a cup of starter.
The starter mixture is meant to feed and fuel growth of wild “yeast”—actually a blend of various species of yeast and bacteria. Some of this yeast is in the air—especially if you’re working somewhere you’ve already made sourdough—but most is in the flour itself.
Within a few hours to a few days, the mixture should bubble and smell sour. If the mixture crusts over before that, just stir it up. If it picks up the wrong microorganisms and smells foul, throw it out and try again.
Some flour may give you a harder time making the starter because processing may have killed or removed the wild yeast. This can be true even if you grind your own—say, with an impact mill.
Your best bet may be stone‑ground flour, because it stays coolest during milling. You can also try flour from different whole or refined grains. Some people say whole rye is the best source of all for wild yeast.
If you get really desperate, you can always buy some starter! You should only have to do it one time.
Once the mixture bubbles, put your starter in a loosely covered jar or crock—no metal—and refrigerate it. Don’t worry about “feeding the starter” to keep it fresh. Left alone, it will stay good for at least two months. When you’re ready to use it, just pour off any black liquid that has formed on top.