Turn the burner under the canner to its highest heat setting, cover the canner with its lid and heat. until the water comes to a full boil. If the jars were pre-sterilized, boil the jam or jelly gently for. 5 minutes. If hot, clean jars were used, process the jam or jelly for 10 minutes. (If you are at an. altitude of 1000 feet or more, add 1 minute of sterilizing time for each 1000 feet of altitude.) The. water level in the canner should be 1to 2 inches above the tops of the jars.