Probably not the "best" way, but a low tech way would be to make a "hominy block" like the pioneers used. It was a 2-3 foot tree trunk with a hollowed out place, deeper in the middle. Corn was pounded into grain with heavy poles, mauls or stones.
Could you improvise with a cast iron dutch oven put inside a plastic bucket for the hollowed out log and maybe use something like a heavy splitting maul for a pestle? (the maul to be used in an up and down motion)
Imagine if you got the shells crushed they could then be put through a Corona grain mill. Would take massive muscle though.
Think I'm missing some important use for ground walnut shells. LOL MissTina, complete my education here or I will wonder every time I throw out walnut shells!!!