Fill a boiling water canner or large pot with water and bring to a boil. Sterilize nine, 125 mL canning jars in the boiling water for 10 minutes. Remove the jars and boil the snap-top jar lids for 5 minutes.
While you wait for the water to boil to prepare the jars, place the sugar, cranberry juice and sparkling apple-cranberry juice in a medium-sized pot and set over medium heat. Heat and stir until the sugar has completely dissolved. Increase the temperature to high and bring the cranberry juice mixture to a boil. Boil hard for one minute, then remove from the heat and stir in the certo. Carefully skim any foam that rises to the surface.
Carefully pour the mixture into the sterilized jars, leaving a 1/4 inch headspace at the top. Wipe the rims clean; centre on the lids. Apply the jars' screw bands until fingertip tight. Process the sealed jars in boiling water for 10 minutes. Remove and cool at room temperature. Check the seal; a properly sealed lid curves downward. Store the jars in a cool, dark place; refrigerate after opening.
Note: Martinelli's sparkling juice is available in the aisle other juices are sold.