If homemade jelly doesn't set up, boil it again with a little lemon juice. For a no-boil option, add liquid pectin according to the package directions instead of boiling.
When boiling jelly a second time, use a large pot. Boiling jelly expands significantly, and a large pot keeps it from boiling over onto the stove top.
Boil the jelly
For each two cups of jelly, add one tablespoon lemon juice. Stir constantly while bringing the jelly to a rolling boil.
Test the temperature
Use a candy thermometer to make sure the jelly has reached 220 degrees Fahrenheit.
Process the jelly jars
Turn the heat off, skim off the bubbles and pour the jelly into clean jars. Wipe the jar rims, put on the lids and submerge the jars in simmering water for 10 minutes.