Basically, you make a sourdough starter and let it ferment. A starter can be 1 cup water or potato water (better), 1 cup flour, 1 tbs yeast. Put in a large jar and cover loosely. Leave on counter and it will begin to bubble. It does best at about 70 degrees. A coffee filter secured with a rubber band works or you can even get a pickle pipe which works super well. Keep an eye on it as it will expand greatly. Leave some in the jar and refeed to restart the process.