Remove grapes from vines and wash them thoroughly with cold water.
To a large pot, add the grapes and 1 gallon of spring water (like Arrowhead or Crystal Geyser).
Bring to a boil, then reduce the heat to medium. Boil for 45 minutes.
About half way through, use a potato smasher to smash the grapes. After 45 minutes of boiling, taste the juice and add sugar if desired. I recommend no more than 1/2 cup of added sugar (unless you like it VERY sweet, feel free to add more).
Let it cool for 30 minutes, then strain a few times into a large bowl. Be careful- it may splatter and stain light surfaces, so keep some paper towels nearby.
Use a super fine strainer if you want your juice completely smooth. I don't usually find this step necessary, but some may not like the mouthfeel. *Note: if using the grape juice to make popsicles or ice cream, don't strain the juice!
Store in an airtight glass container. You can keep this in the fridge for 7-10 days.
I recommend drinking the grape juice with lots of ice! So delicious and refreshing :)