Combine grape juice, lemon juice, and sugar in a flat-bottomed kettle. Bring to a rolling boil. Add liquid pectin, and return to a boil. Boil 1 minute, stirring constantly. Remove from heat, and skim off foam with a metal spoon.
Ladle hot jelly into hot sterilized jars, leaving 1/4-inch headspace. Cover with metal lids, and screw bands tight. Process jelly in boiling-water bath 5 minutes.