Combine grape juice and pectin in a large saucepot. Bring mixture to a rolling boil. Stir in sugar and return to a rolling boil. Boil hard for 1 minute, stirring constantly. Remove from heat. Skim foam if necessary. Pour into hot, sterilized jars, leaving 1/4 inch headspace. Adjust caps. Process 5 minutes in boiling water bath. Makes 5 pints.