Put grape juice in a large saucepan. Add sugar, stirring until dissolved. Bring to a boil over high heat, stirring constantly. Stir in liquid pectin. Return to a full rolling boil. Boil hard for 1 minute, stirring constantly. Remove from heat. Skim foam if necessary. Ladle hot jelly into hot jars, leaving 1/4-inch headspace. Adjust two-piece caps. Process for 10 minutes in a boiling-water canner.
How to Make Juice for Jelly from soft fruits such as Grapes.
Select top quality fresh picked fruit, if available. Wash and stem fruit. Place grapes in a large saucepan. Slightly crush fruit using a potato masher, add a pinch of salt.
Add a pinch of salt and 1/4 cup water for each quart prepared is an optional step. I prefer not to add any water.
Cover; simmer until fruit is soft. Strain mixture through a damp jelly bag or several layers of cheesecloth to extract juice. Juice may be used fresh, canned or frozen for later use.