Pick the grapes from the stalks. Wash thoroughly and put in a jam pan or large saucepan. Sprinkle the sugar over the grapes and leave for 8-9 hours or overnight.
Add the glass of water to the grapes. Bring the grapes to the boil, simmer for 5 minutes, then remove from the flame and leave to cool.
Return to the heat, bring back to the boil and simmer for another 5 minutes. Remove from the heat and allow to cool. Repeat the procedure once more, bringing the fruit to the boil 3 times in all.
By the end of the cooking process, the grapes should be the colour of amber and the jam the consistency of honey. The grapes should retain their shape.
Place the preserves into sterilized jars and seal.