As far as feeding the egg shells, once crushed, just put some out and let them have what they need. I like using the food processor for the shells and you don't need to crush them down to much. We eat lots of eggs here at home and we collect the shells so we have quite a bit. I find the processor easy and quick for the quantify we have. But whatever works for you is just as good. If you want to use for your garden however; you might want to crush the shells down to small bits and mix with compost. Just great for the garden. Between our chickens and composting we have very little to place out in the trash. When our dogs were pups we gave them the barf diet and mixed in the shells finely ground. The Barf Diet is a meat and some other natural items.
That is the best thing about chickens. They eat everything that is left over or I don't want to eat so I don't feel bad if I didn't get through all of the lettuce in the fridge.