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Store mussels in a colinder covered with a damp cloth in the bottom of the fridge, and use within 24 hours. Do not keep in fresh water or the mussels will die.
Check that all the mussels are closed. Give any that aren’t a gentle tap and discard those that don’t close, along with any that are cracked or damaged.
Scrub the mussels well with a stiff brush under cold, running water to remove any barnacles. Pull away the tendrils or ‘beards’ from the sides of the shells.
Cook the mussels according to your recipe and discard immediately any shells that have not opened up.