Add the walnuts to one of the cheesecloth bags. Choose black walnuts that have fallen from the tree, but are not yet ripened.
Lay the bag into the bottom of the cardboard box. Let it sit outdoors in the box for two weeks.
Smash the walnuts, while they are inside the bag, with the hammer. Do this with the bag still inside the box.
Pour the bag into the box, and sift through it to remove the walnuts from the cracked hulls. Wear gloves as you do this because the black pigment will stain your hands.