Wash grapes and place them without any water at all, in a pan and mash them down with a potato masher.
Bring them to the boil, stirring frequently and boil until pulped.
Strain through a sieve pressing down hard with the back of a spoon.*
Measure the liquid and add cup for cup of sugar
Boil as hard as possible or until a little on a saucer will set.
(To obtain a perfectly clear jelly I used a double strength muslin bag to strain pulped grapes. As I used the old preserving pan and had a large amount of grapes in the pan I would leave it to strain overnight).