Sieve the flour, cornflour and salt together, make a well and add the oil and about half the water. Mix into the flour and continue adding more water until the dough is smooth and does not stick to the hands. The dough should then be covered with a damp cloth and left to rest in the refrigerator for a minimum of 2 hours. (The paste will not be elastic enough if left for a shorter period.) I prefer to leave the paste for a longer period, and find 24 hours about right. It will keep for up to 2 days in the refrigerator, as long as it is covered with a damp cloth.
To use, roll out the pastry as far as you can, using plenty of flour on the work surface at all times. When you have rolled it out as thin as possible, place the sheet over the backs of your hands and pull gently downwards, stretching all the time, until you can virtually see through it.
Once rolled out, the pastry should be used straightaway.