Get the how-to for the Jelly Labels.
Whisk together pectin and 1/4 cup sugar in a small bowl, and set aside.
Pulse gypsy peppers with red-pepper flakes in a food processor until finely chopped. (Mixture will measure about 2 1/2 cups.) Stir together pepper mixture, vinegar, butter, salt, and remaining 3 cups sugar in a 5- to 6-quart heavy pot. Bring to a vigorous boil over high heat, then continue to boil vigorously, stirring occasionally, 5 minutes. Gradually add pectin mixture, whisking constantly. Return jelly to a vigorous boil, stirring constantly, and boil, stirring constantly, 1 to 2 minutes (mixture will thicken slightly). Remove from heat. Season to taste.
Spoon into hot sterilized jars using a sterilized spoon, cover with hot sterilized lids, and can in a water bath (directions follow).
Wash jars, lids, and screw bands in hot, soapy water; rinse well. Submerge jars upright on a wire rack in a large pot of hot water, and bring to a boil. Boil for 10 minutes. Remove pot from heat, and let jars stand in water until ready to use. Sterilize lids in simmering water, and let stand in water until ready to use. Remove jars and lids from water using tongs, and transfer to a layer of clean towels to air-dry before filling.
Once jars are filled, wipe rims and then close with screw bands and lids. Submerge jars, covering with water by 2 inches, upright on a wire rack in a large pot of hot water; bring to a boil. Boil for indicated time (or longer, depending on altitude). Remove jars from pot using tongs, and transfer to a layer of clean towels to cool. Once cool, check for a slight indentation in lids, which indicates a vacuum seal. Refrigerate all canned products after opening.