Pour 20 oz of boiled wort into a container, add a handful of crushed two row, cover and leave for a week or so. Strain off the grains, then boil for 15 min. Add to the fermenter. I did O'Flanagan's sour, it is excellent, although I plan to sour about 30 oz when I next make a dry stout using 70% pale, 20% flaked barley, and 10% roast barley.