I had about two gallon zip bags of shells (why I always buy shell on). I roasted them on sheet pans in the oven, for maybe 15 minutes.

Into a pot with cold water, an inch above them.

Bring just to not quite a boil, add some onion, carrots, & celery (also formerly in the freezer from cuttings), and simmer maybe 45 minutes more.

Lot of recipes call for white wine, but I never have any.

I’ve done it with crab shells added before, but rarely have those, and didn’t notice much difference.