"Your first task is to wash away the mud and/or sand. This is easily done with a soft flush of water above a small wire-mesh: in this way the risk of loosing small specimens is limited.
Then, you sort them out according to size: all shells below 4 mm and best be kept in a small glass vial with alcohol, for a few hours or even a few days before drying them out.
With a knife or a needle (according the solidity of the shells) you can remove the majority of calcareous algae, barnacles or worm tubes.
To remove the mollusc from your shells there are various options:
Freezing : put your shells in the freezer for a couple of days. Afterwards, you can pull out the animal with a bent needle or tweezers.
Boiling : slow boil your catch for 20 minutes. Again, it will be easy to remove the animals with a knife, a needle or tweezers.
Lastly you can soak your shells in 50 % bleach and water. This will help dislodge the more stubborn encrustations.
After drying, the aperture can be filled with cotton-wool on which you can fix the operculum with an easy-to-remove glue. Some shells are dull and lifeless after cleaning: a thin layer of mineral oil of glycerine will give them back their sheen and colour.
There is a special treatment for POLYPLACOPHORA: after collecting you can tie them on to a flat piece of wood. After a few weeks of drying out, they are ready for the collection.
From Poppe & Goto, European Seashells Vol. I.