OK. I think you need to wash them in several changes of cold water then soak for a few hours in cold water.
Then most recipes suggest boiling in their shells for up to 45 mins. I found one recipe which suggests only cooking for 15 mins to avoid them becoming rubbery. I would try that.
You then need to pick them out of the shells and remove the guts. Its the muscle you eat. You can then slice them up and fry them quickly with garlic. Alternatively, dip pieces in flour, egg and breadcrumbs and fry.
Please note I've never cooked them myself! They are a bit similar to abalone - which is much prized in the USA. So it might be worth looking at recipes for abalone.
I tried buying a pound of whelks at a market when I was visiting Paris in response to a thought that went something like, "These look amazing and intimidating and hence I must try them." Anyway, best of luck with them because what I ended up making with them was not particularly appetizing. Caramelizing minced shallots with lemon and molasses works miracles with clams and mussels but somehow loses its magic with whelks.