Harvest black walnuts as soon as the outer husk softens, but is still green. The best quality nutmeat is light in color and milder in flavor. If you can leave a finger depression in the husk, the nut is mature. Most people wait until nuts start to drop before gathering. However mature nuts can be shaken from tree limbs or dislodged with a long pole.
Proper gear is important in walnut handling.
The outside husks will stain just about anything. Walnuts were used in the past for dying cloth and baskets. You may even want to wear your favorite football or bike helmet. Style isn't the goal here. There are many ways to remove green or partially decomposed husks.