Hull black walnuts by rolling them under your foot until the hull falls off or scattering them on gravel and driving over them. A corn sheller or mechanical devices designed to remove the hulls from black walnuts work as well.
If you have only a few walnuts to hull or lack a mechanical device, you can drill a hole in plywood that is just smaller than the hulled nut, about 1 5/8 inches in diameter. Place the nut over the hole, and hammer at the nut. This forces the shelled walnut through the hole while forcing off the husk. It is important to remove the hull as soon as possible as the black dye in the hull and the bitter flavor can seep into the nut if left on for too long. Before hulling black walnuts, wear heavy work gloves to protect your hands from both the hard shells and the dye.
Once a black walnut is hulled, it still contains a moist shell that needs to be dried and cured or cracked immediately. Rinse the shells with water, then spread them out in a shaded area for several weeks. Once the unshelled nuts dry out, store them in a cool, dry area. You can also crack the walnuts immediately, but the meat needs to dry out before being stored or eaten.