Wash two jars (250 ml each) thoroughly with soapy water. Then sterilize jars in boiling water for 8 minutes. Also, boil the metal lids. Dry on a clean towel. Let air dry naturally. Alternately, you can place washed jars while they are still wet in the baking tray. And roast jars in a preheated oven (160-180 C) for 15 minutes.
If you are making grapes jam with seeds then deseed by cutting grapes in half and removing seeds with your thumb.
Transfer grapes to a food processor and blend until smooth.
Transfer grape juice to a wide shallow pot and add sugar, and lemon juice (also add half a cup whole grape for chunky jam, optional)
Cook jam for 30-45 minutes on medium-low heat until jam is thick and glossy.Protip: The jam will splutter a bit when closing the setting point.
Do a cold plate test by dropping a teaspoon of jam on a cold plate. Draw a line. The line shouldn't meet.
Transfer hot jam to clean dry jars seal the lids and turn the jars upside down if storing jam in a fridge or freezer. Let it set to room temperature for 12 hours then transfer to fridge or freezer.