In the fall, black walnut hulls will change from green to light yellow or brown indicating that the walnuts are ready to harvest. You can pick mature black walnuts from the ground beneath the tree, or if the tree is small enough, you can shake the walnuts from the tree. Harvesting the nutmeats inside the walnut shells does take a bit of effort, but the results are well worth the energy.
Collect black walnuts from beneath walnut trees in a paper sack.
Remove the outside hull by pulling the yellow/brown husk away from the inside nut.
Place the nuts in a pillowcase (or other cloth bag) and tie the top of the pillowcase closed.
Hit the black walnuts inside the pillowcase with a hammer to break the shells. It is best to do this on concrete, such as a driveway or sidewalk.
Remove the walnuts from the pillowcase and pick the nutmeats from the broken shells.