Recommendations for jelly making have changed in recent years. Many old recipes advise sealing with paraffin or filling and sealing the jars without processing, but this is no longer recommended. Current guidelines require sterilizing the jars by boiling for at least 10 minutes before filling and then processing them filled for 6 minutes.
The best jars for making jelly are Mason-type jars with self-sealing lids - the boiling directions that follow are for 1/2 pint jars. Wide-mouth jars are easier to fill and clean. Jars are expensive, but they last for many years.
Clean the jars in the dishwasher or by hand with hot soapy water and rinse well. Place them upright on a rack in a boiling water bath canner or large stockpot. Fill the canner with hot water, covering the jars by about 1 inch. Bring the water to a boil and boil for at least 10 minutes at a full boil. Allow 1 extra minute of boiling time for every 1,000 feet of altitude over 1,000 feet. Remove the jars and allow them to drain before filling. Jar lid preparation varies with manufacturers; prepare the lids according to the package directions.
Fill the jars with jelly, allowing 1/4 inch of empty space at the top. Wipe the rims and the threads of the jars with clean paper towels to remove any spilled jelly. Place a prepared lid and screw band on each jar and hand-tighten the band.
Return the filled jars upright to the rack in the canner or stockpot. Make sure they are covered with at least 1 inch of water and bring the water to a boil again. Process jelly for a full 6 minutes after the water reaches a boil. Again, add 1 minute for each additional 1,000 feet of altitude.
Jars do not need to be pre-sterilized if they will be processed in a boiling water bath canner for at least 10 minutes after filling. You can skip the first boiling step by increasing the processing time to at least 10 minutes after filling the jars. Add extra minutes for high altitude. Larger jars will need longer; remember these directions are for 1/2 pint jars. Research by the University of Minnesota showed that there was no harmful effects when processing up to 15 minutes.