To make Certo grape jelly, gather canning supplies, 3 pounds of Concord grapes, 1/2 cup of water, 7 cups of sugar and 1/2 tablespoon of butter. Have a pouch of Certo fruit pectin available as well.
Fill a canning pot halfway with water, and bring it to a simmer. Wash all of the jars, lids and screw bands in hot soapy water, and rinse them before sitting them aside. Crush and stem all of the grapes, and place them in a saucepan. Add the water, and bring the pan to a boil. Reduce heat, and simmer the pan for 10 minutes, stirring occasionally. Pour the fruit into three layers of damp cheesecloth, and allow it to drip into a bowl until 4 cups of juice are available.
Stir the sugar into the juice, and add butter to reduce foaming. Bring the liquid to a rolling boil while stirring constantly, and add the Certo. Boil for exactly one minute, stirring constantly. Remove from the liquid from the heat. Skim off the foam before putting the jelly into jars. Leave about 1/4 inch at the top, and wipe the rims before putting on the lids.
Place the jars on the canning rack, lower the rack into the canning pot, and cover the pot with water, making sure the jars are about 1 inch under the surface. Bring the water to a boil, and wait five minutes before removing the jars. Check the seals of the jars, and store them.