Begin by sifting the icing sugar (all but 1 heaped tablespoon) into a large bowl and making a well in the centre.
Add the water, a teaspoonful at a time, and stir with a wooden spoon until it is too stiff to stir any more. Then, using your hands, form the icing into a ball before turning out onto a clean, smooth, dry surface dusted with the reserved icing sugar, and knead for about 3 minutes until it becomes completely smooth (if the icing is sticky, knead in a little more icing sugar, or if it is dry run a little cold water over your fingers).
Then roll out the icing onto a clean surface dusted with a mixture of icing sugar and cornflour, and keep giving it quarter turns after each roll. Once it is rolled to the right size, ice the cake as soon as possible.
If you want to make it ahead, it can be kept tightly wrapped in the fridge for 3 days (though once it is on the cake it does not need to be refrigerated).