Cleaning and preserving a dead seashell requires removing the barnacles, bleaching the seashell and polishing it to a nice shine. Dead seashells need to have the tissue removed first, which is done with freezing, boiling or burying.
If someone finds a seashell that is still living, there is tissue inside that must first be removed. Burying is often the preferred method, though it takes longer. Find an area where a hole can be dug in the yard that isn't going to be disturbed by pets or other animals. Bury the seashells 18 inches deep, and keep them there for a few months. This is long enough for insects and worms to remove the tissue. Freezing works by placing the seashells in a freezer bag, filling the bag with water, and freezing it like fish. When it is time to clean the shells, thaw the bag at room temperature, then grab the animal tissue in the shell, and remove it.
For dead seashells, cleaning doesn't require these other steps. Make a solution that is half water and half bleach. Place the seashells in the solution, and let them soak. When the leathery covering is gone, it is safe to remove the seashells from the bleach. Chip off any barnacles on the seashells, and clean them with a wire brush or toothbrush. Use baby oil to polish the seashells.