Ingredients:-
1.25 kg grapes,
6 cups granulated sugar,
Juice of half a lemon,
1/2 tsp unsalted butter,
Jars:-
Have ready: 6 x 237g sterilized jars and lids.
METHOD:-
Separate the grape skins from the pulp by squeezing the grapes between your fingers. Put skins in the food processor bowl and pulse until they are coarsely chopped. Place them to a large heavy-bottomed pot and add 1/4 cup of water. Bring to a simmer and cook until the skins have softened a bit, about 10 mins.
Place the grape pulp in a medium saucepan and bring to a boil. Reduce the heat and simmer until the grapes lose their shape, about 10 mins. Pour the grape pulp through a fine mesh sieve into a large bowl. Force out as much pulp as you can and discard the seeds.
Add the grape pulp to the pot with the grape skins and stir to combine. Add sugar, butter, and lemon juice and bring the mixture to a boil, stirring constantly. Reduce the heat and simmer, stirring frequently, until the jam has reduced and thickened and reached the gel stage, about 45 mins. Test the jam, put a small plate in the freezer. If the jam stays put and doesn't run, it's set. If not, simmer the jam for a few mins more.
Spoon the hot jam with a ladle into hot sterilized jars, leaving 1/4-inch headspace. Wipe rims of the jars, cover with lids, and screw bands on until just barely tight. Place jars on rack in pot and cover completely with water. Cover pot and bring to a boil over high heat. Boil for 10 mins. Turn off heat, uncover pot, and allow jars to rest in water for five mins. 
Remove jars from pot and allow them to rest undisturbed on a bench for six hours or overnight.