I did it once. With a pillow case and a hammer.
We have access to lots of oysters. It seemed like a good idea at first...
But it stunk, it took forever, it was hard work, and it was messy. Needless to say, I happily buy mine from the feed store.
I get a 50-lb. bag for like 6 bucks, so it wouldn't be worthwhile for me. Your mileage may vary. I'm pretty sure that my little crank grinder would handle the job if I smashed the shells up with a mallet first, but as I already use it for grinding my own feed, that would be just too much work for me.
I would dig a hole in the ground and place a scoop of gravel down there and a good size round from a tough tree (Oak or whatever). If the top of the stump were about a foot deep or so that would seem about right. Then I would take my steel tamping rod and crush the shells down on top of the stump in the hole add more, scoop them out with a shovel into a can as needed. A modern version of a mortise and pestle.