The only really difficult aspect of collecting black walnuts is outwitting the neighborhood squirrels, who have all day to bide their time and watch the tree as fruit begins to ripen and drop to the ground beginning in early September. Fortunately, black walnut trees usually produce enough delectable nuts to go around. There's no point in trying to beat the squirrels to the harvest by pulling black walnuts from the tree prior to September. This will damage the stems that bear fruit forming for the following year's crop. Besides, they aren't ripe yet, anyway.
Begin watching your chosen black walnut tree in early September. Check the ground each day for fallen fruits and collect them in a bag. They'll begin ripening from now through early October, and will drop easily from the tree when mature. Each black walnut is encased in a tough, thick, green hull that's about 2 inches in diameter and resembles a little basketball.
Squeeze the green nut hull firmly between your thumb and fingers to make little dents in it. If the dented spots don't spring right back into position without leaving a mark behind, they're not ready. Immature walnuts won't ripen further once they've fallen from the tree, so toss them to the side for the squirrels.
Look the hull over for holes, indicating insect grub damage. Toss those over for the squirrels, too.
Put on a hard hat or bicycle helmet and bump fruit-bearing limbs of the tree firmly with a long stick or piece of PVC pipe to knock black walnuts to the ground. Do this every day beginning in mid-September, when great volumes of fruits are ripening simultaneously.