Wash mussels with cold water, check if there is any one already open. If the mussels is opened and won't close when pressed, that means it is not alive and thus is not edible. Closed oyster shells should be smoothly penetrated with a knife, separating the "meat" from the shell. Season the "meat" with lemon juice, olive oil, freshly ground pepper and serve in the shell half.
Do not salt mussels. If shellfish have an unpleasant odour, discard them.