The first thing to do is choose and buy your cooked crab.
The fishmonger will pull it slightly open so that you can see it's full of meat and not empty. Back in your kitchen you'll need a chopping board, two bowls (one for the white meat and one for the dark meat), a small sharp knife, a teaspoon, a metal skewer (preferably flat), and either a small hammer or a pair of nutcrackers.
Put the crab on its back on the chopping board, so that the claws and softer body section face upwards, then simply twist off the legs and claws – they'll come away very easily – and put them to one side.
Now put your thumbs against the hard shell close to the crab's tail, and push and prise the body section out and away from the shell.
From the shell remove and discard the small greyish-white stomach sac, situated just behind the mouth.
Remove the long white pointed 'dead man's fingers' from the body section. These can be easily distinguished and it is a quick and easy job to remove them.
The body section (and in particular the parts where the legs and claws joined the body) is a mass of tiny crevices, all harbouring delicious meat. Scrape and pick the meat out, dividing it between the bowls according to the colour of the meat. Remove the meat from the shell in the same way.
Crack the claws and legs with your hammer or nutcracker then, with a skewer, poke out all the white meat into the appropriate bowl.
If you want to serve the crab in the shell, break off the jagged, overlapping rim from all round the edge of the shell, then wash and dry it well, and smear oil inside.
Season the white meat with salt, freshly milled pepper, cayenne and lemon juice (chopping or shredding the meat a little). Now season the brown meat in the same way. Place a strip of parsley down the centre of the shell, and place white meat on one side of it and brown meat on the other. Alternatively, place brown meat down the centre of the shell, and place the white meat on either side. Finally decorate the crab with finely chopped parsley and serve with a fresh crisp salad and some home-made mayonnaise.