How do you know the walnuts are ready to pick?
I live in an area where we have black walnuts. They are harvested after they fall from the tree in the fall. Other walnuts have the same basic harvest time. You need to pick them up to dry fairly quickly to prevent decay and discoloration. There is an Oregon State guide for nut harvest that should give you the information you need to harvest, condition and store your nuts for your use over time.
How to harvest, handle and store nuts from the home orchard