In a medium saucepan, stir together the chopped peppers, granulated sugar, pepper flakes, and salt. Let the mixture sit at room temperature for 30 minutes so that the peppers begin to release some of their juices.
On medium heat, bring the mixture to a simmer, stirring occasionally. Make sure all the sugar is dissolved before proceeding.
When the sugar is dissolved, add the lemon juice and liquid pectin, and stir. Continue simmering for another 10 minutes
Add the cranberries, and simmer gently for another 10 minutes or until the cranberries have burst and the mixture has thickened slightly.
Transfer jelly to sterilized jars, and let cool completely before refrigerating. Alternatively, you can seal the jars according to your jar manufacturer’s instructions.