Wash, stem, and crush grapes; add small amount of water and boil 15 minutes. Press through jelly bag and strain. Measure juice and bring to boil. For each cup of juice add 3/4 cup sugar. Boil rapidly to jelly stage. Pour into sterilized jelly jars. Fill to within 1/2 inch of top. Put on cap, screw band firmly tight. Process in boiling water bath 5 minutes. Crystals often form in grape jelly. To prevent this allow extracted juice to stand overnight in a cool place. Next morning carefully pour juice off, leaving sediment in bottom of pan.