In case you prefer the light-colored kernels, hull the walnuts immediately after they drop from the tree, because allowing the hulls to partially decompose before hulling may cause a discoloration of the kernels.
The hulls are thicker and fleshy at maturity. They could be mashed and removed by hand, but of course, using some mechanical devices such as a corn sheller make the job much faster and way more convenient.
Kernels that have been tempered before the shell is cracked are easier to remove. Soak the nuts in water for few hours, drain, then keep in a closed container for about ten more hours. The kernels should then absorb enough moisture to become tough, but still will remain loose in the shell.