What you need:
One small crab per person. This will yield two large pincers, one, um, whatever the central body part is, and six narrow lesser 'legs'.
One small red onion.
One small (plum) tomato.
Ginger, pasted. Garlic too, but it's optional.
Cumin and coriander powdered or pasted fresh.
Lots of groun black pepper -- essential!
Wash and set out the crabs to dry. They will look quite creepy. Ignore your gut. If they're still alive, keep them in hot water for a while (like you'd cook lobsters, but minus the boiling).
When dried, rub them with turmeric and salt. Sauté or shallow-fry in a wok till the shell turns bright orangish and white. Drain and keep aside.
Now do the same fryin/draining routine with halved or quartered potatoes. Grate the onion so it yields a slighly watery, pungent paste. Chop the tomatoes till they're tiny. Don't throw away the soft cores. Scoop them and keep them in a bowl/saucer, if that helps with the chopping.
Now, in the same wok, heat a little more oil, if needed. Add the pasted ginger (and garlic, if using). Fry them till they change colour and start smelling fried. Then add the grated onion and stir quickly over high till it slowly starts smelling fried as well. Them add the tomatoes, and the soft cores if you've kept it separate. Add salt. On high, keep stirring till everything mixes well together and forms a oniony, gingery, tomatoey paste and the tomatoes are no longer raw (their colour will change to a darker red). Lower the flame. If using coriander and cumin powder, mix them with a little water to make a paste and add the paste to this mixture. Top with as much sugar as you prefer. Again, fold the entire thing in well. If you like your food super spicy, this is the time to add red chilli powder or chopped green chilies.
Now add the potatoes and fold in well. Do the same with the crab. Cook on medium for about three or four minutes to let the flavours seep in. Keep stirring so the bottom doesn't char.
Then, add enough water to submerge half the crab(s). Cover, simmer and cook for about twenty minutes, or till potatoes are completely tender.
Turn off the flame. Liberally pour ground black pepper over the curry. Mix it in while the curry is still red-hot. Cover again and keep the pot till it's time to serve. Best accompaniment, white rice :-)