we tried this. The resultant mess was revolting, and we threw it away. We're still searching for a decent sourdough starter recipe.
1 bunch of organic grapes, 500g unbleached strong white bread flour (preferably organic), 1 litre of water.
wrap the grapes in muslin, tie with an elastic band, and bash them with a rolling pin.
mix the flower and water together in a large pan or whatever, squeeze in some grape juice, then submerge the muslin bag'o'grapes in the floury mess.
cover and leave for about 10 - 14 days, until the grapes start to ferment, and the cloth inflates slightly with the gasses (sounds lovely, doesn't it?!)