No need to break them on a rock. A little opposing pressure on the two halves of the shell will allow you to slip a knife in between the shells

A quick side to side with the knife will cut the white muscle that holds it shut .

I always put the knife in on the flat side of the shell .

Either slide a few onto a bait needle and bind it up or just hang the item on a hook and bind it up . Makes for a messy bait but the fish dont seem to care

Broken shells are very sharp so be careful if you smash them up