The outer shell (carapace) of the common shore crab Carcinus maenas needs to be cast if the crab is to grow. During this stage the crab becomes known to anglers as peeler crab and to cod as dinner. I have been told that fish in aquariums seem to know when the carpace is due to come off and hang around the rock impatiently, it is rumoured the crab always leaves it's shelter after casting and seeks new shelter.
Look for crabs in areas with lots of rock and weed, children seem to have an uncanny knack of knowing which rocks to look under. Look for medium sized crabs, large ones moult less frequently and small specimens should be left till another day. Crabs that are ready to peel will have a hairline crack near the tail end, be dull in colour with a soapy feel. A discarded shell nearby often indicates a freshly peeled crab is nearby. Those not quite ready can be hurried along by storing in seaweed (change regularly) in a cool spot. Never keep soft crabs in the same container as peelers. Leave females with eggs alone. If you intend to lay a bit pipe to catch crabs put it in an difficult to reach place out of site to prevent your crabs from been stolen.
To use remove the legs (use to make cocktails), bung on a hook and tie with thread (elasticated bait thread is best but ordinary will do). Large specimens can be cut into smaller pieces. To freeze crab remove the lungs first, I like to freeze them on the hook and pack individually, store in a thermos flask when fishing.