The berries ("grapes") are very tart but make delicious jelly. They're free, too!
Rinse them, then put them in a pan. Mash them, then add water, 1" deeper than the berries. Bring to a boil, then reduce heat and simmer, covered, for about 20-30 minutes, until they're very soft. Strain through a fine-mesh sieve, a few layers of cheesecloth, or a single layer of cotton fabric. Let it drip for ten minutes or so. You can press down on it with a spoon to get more juice, but this will squeeze more solids into the juice, yielding cloudy jelly.
Follow the directions for grape jelly from your brand of pectin, or use these quantities:
Combine 5 cups juice with 7 cups sugar, bring to a full boil for two minutes. Stir in one box of pectin (THIS WILL FOAM UP!- use a bigger pan than you think you'll need.) and return to a full boil. Let it boil 2 minutes more, then pour into your sterilized jars. Top with new lids (warmed in water), screw on the bands. Process for 10 minutes. This makes 7 tp 8 half-pints of jelly. If you used a big enough pan, you can scrub it out, put 1" of water in the bottom, bring it to a boil, put the jars full of hot jelly into this pan, cover with a lid, and let it steam (simmer) for the ten minutes of processing time. Lift out carefully and put them on a dry dishtowel on the counter. Cool completely.
When the jelly is completely cool (usually the next day), wash the jars and remove the rings. Dry the tops, label with the year and what's in the jar.
For more detailed instructions, see a post at one of my favorite websites, http://www.pickyourown.org/grapejelly.htm