With a crafting table.
I'd say the best way to get the shells are fishing, but if you have a drowned farm for copper and/or tridents you might get some from there too.
For the Heart of the Sea, go to a shipwreck and find a treasure map. You can bribe dolphins with fish and they'll lead you to the shipwreck/drowned town and then you can find a treasure map there. The treasure will be somewhere semi-close to where you found the map, head to the location and find the treasure (There are some tricks to finding it if you need them) In the treasure chest there will be a Heart of the Sea 100% of the time.