If you don't have a juicer I know that there are other ways of doing it. Just google Juicing grapes and it will take you there.
The kind of juicer I am using has 3 levels. the top is the basket (Kind of like a big colander) this holds your grapes. The middle level is where the juice drips into. The last level is full of simmering water. you bring the bottom pan with water to a boil and then turn it down to a simmering boil. This steams the grapes that are up in the top level. As they soften the juice seeps out into the middle pan. Then the drains out of a spout in the the middle level. You attach a rubber hose onto the spout and you can place the hose into whatever container you are wanting your juice in. As you can see from this picture I have mine draining into a gallon milk jug that I have cleaned out and sanitized.
So as you can see the juice is happily draining in there. I leave some space on top because once it freezes it swells up some.
So there you go! This juice is ready to cool and go into the freezer.
I pull the jugs out of my deep freezer when I need them. I let it thaw in the fridge and I can make about 4 pitchers of juice from this gallon of concentrate.
How to mix your Grape juice concentrate
1 quart of grape juice
1 quart of water
1 cup of sugar (I like to do 3/4 cup)
stir and enjoy :-)