Put a hot water bath canner on the burner and fill with enough water to cover jelly jars, with an extra inch of water over the top of jars. Turn the burner on high and cover with the lid.
Wash jelly jars and screw bands. Place jelly jars in the canner to sterilize.
When the water in the canner comes to a full rolling boil, boil jelly jars for 20 minutes to sterilize them. Set the timer and start making your jelly.
Prepare canning jar lids according to the manufacturer's directions. (Most jar lids are no longer designed to be boiled, be sure to check.) Pour boiling water over the screw bands and set aside.
Reconstitute frozen cranberry juice and measure 6 cups of juice into a large saucepan.
Measure the pectin into a small bowl and add 1/4 cup of the sugar to the pectin. Combine and set aside.
Measure the rest of the sugar into a bowl and set aside.
Place the saucepan of cranberry juice on a medium-high burner. Stir the juice as you bring it to a full rolling boil.
Slowly add sugar and pectin mixture and stir to incorporate.
Bring juice mixture back to a full rolling boil, then add the remaining sugar all at once and stir.
Bring the juice back to a full rolling boil. Stir and boil for 1 minute or until jelly forms a sheet on a cold spoon. Turn off the burner.
Use a jar lifter to remove sterile jelly jars from canner and place them on a heavy towel on your countertop.
Place a canning funnel in a jelly jar and use a ladle to fill with jelly. Work quickly but carefully to prevent the jelly from cooling.
Wipe rim of jar clean, place canning lid on the jar, then screw canning band in place until finger tight.
Use the jar lifter to place the filled jar into the boiling water bath.
Repeat until all of the jars have been placed in the canner. Place lid on the canner and return to a full rolling boil.
Process jam for 10 minutes once the water returns to a full rolling boil. Remove jars from canner with the jar lifter and place them on a heavy towel to protect your countertop. Allow jars to cool without disturbing the jars or screw bands until jam reaches room temperature.
When the jelly has cooled completely, remove screw bands and wash the jars to remove any jam that may have leaked during processing. Check each jar to be sure it has sealed properly. A properly sealed lid will not spring up and down when pressed with your fingers.
If any of the jars did not seal properly, refrigerate them and use the jam within 3 weeks.