From The University of Illinois, College of Agricultural, Consumer and Environmental Sciences, Horticulture Solutions Series
To use: Collect the acorns in the fall, when ripe. Remove shells and caps. The shells will come off easier if you first slit with a sharp knife. Boil the acorns whole for at least two hours, changing the water each time it becomes light brown in color. This boiling removes the bitterness and they become pleasantly sweet. You will find, after this boiling, that they are quite dark brown in color. Toast in a 350 degree F oven for another hour. They can then be eaten as they are or ground into a flour.