In a large, deep stainless steel saucepan, place apple juice. Whisk in pectin until dissolved. Stir in the butter.
Bring to a boil over high heat, stirring frequently. Add sugar all at once and return to a full rolling boil, stirring constantly. Boil hard, stirring constantly, for 1 minute. Remove from heat and quickly skim off foam.
Quickly pour hot jelly into hot jars, leaving ¼" (0.5cm) headspace. Wipe rims. Center lid on jar. Screw band down until resistance is met, then increase to fingertip-tight.
Place jars in canner, ensuring they are completely covered with water. Bring to a boil and process for 10 minutes. Remove canner lid. Wait 5 minutes, then remove jars, cool, and store.
Video
Notes
Don't skip the butter! The fat in a small amount of butter (or margarine) helps keep the jelly from foaming.
Test your gel: Place a small spoonful of jelly on a saucer that has been placed in the freezer. Let the jelly cool, then run your finger through the puddle-if it is the texture of jelly, you are ready to can! If it is still too thin, continue to boil hard, testing every minute until it's the right consistency.
Wipe your rims! It is important to wipe the upper rim of the jar before placing the lid on, because any jelly residue between the jar rim and the lid can cause it not to seal properly.