Bring grapes to room temperature.
Sprinkle salt and spray vinegar. Set this aside for 10 to 15 mins. Meanwhile destem the grapes. Discard the rotten grapes, if any.
Add water just enough to cover the grapes. Rub them gently. Discard the water.
Rinse them a few times with lot of water. If you like chilled juice, then chill or freeze them for 1 to 2 hours depending on how cold you want it to be.
If making it in blender, add grapes along with the desired ingredients.
Add water according to your taste, depending on how diluted you want it to be.
Place a strainer over a bowl and pour the grape juice. Using a spoon pass the juice through the strainer. Pour to serving glasses. If you want you can add ice cubes to it.
To make in slow juicer, follow the manufacturers manual & run the juicer. Add grapes to the juicer chute and collect the juice in the pitcher.
Serve grape juice immediately before it begins to oxidize.