Easiest thing in the world.
Wash the mussels and scrape off the "beards" with the edge of a sharp knife. Do not cut yourself. Throw away any that are already open.
Prepare a sauce of enough volume to cover about a third of the mussels. Basic "moules marinere":
Half a bottle of dry white wine, 1 stick of butter, juice of one large lemon, a lot of chopped fresh garlic, salt to taste.
Bring the sauce to a boil, dump in the mussels, cook covered until the mussels open. It only takes a few minutes, don't overcook! You might want to stir the pile once or twice.
Garnish with fresh parsley, basil shallots or whatever.
Serve with hot french bread for dipping the sauce out.
Variations include less wine, no lemon and cream.
Sometimes with onions and tomatoes.
Some like taragon and Dijon mustard in the sauce.
If you're at the beach, all you need is a bag of fresh mussels, half a bucket of fresh seawater, a lemon and a fire. Most traditional.