Acorns

Gather in the fall. Those from white oak trees should not need to be leached to remove tannins, but other kinds do. If in doubt, do it, or they'll be bitter. Place whole, chopped, or coarsely ground nutmeats in a cloth (a clean T-shirt will do) and tie with string. Place in boiling water until water turns brown, then drain, and add more boiling water. Repeat many times as needed until water is clear. In the field, just place bag filled with nutmeats in a clear running stream until nutmeats are no longer bitter, from 1 to several days. Spread the leached acorns in a pan and dry in the sun or in a warm oven. Use like chopped nuts or nutmeal.

From Edible Wild Plants: A North American Field Guide by Elias & Dykeman.