For each cup of juice needed, use about 1 pound (500 grams) of elderberries. Gently wash and drain berries, remove caps and stems, and place berries in a large stainless-steel saucepan with just enough water to prevent scorching (¼–½ cup water per 4 cups berries). Bring to a boil over medium-high heat, stirring frequently. Reduce heat, cover loosely, and simmer 5–10 minutes, stirring and crushing berries until softened. Transfer to a dampened jelly bag or a strainer lined with several layers of dampened cheesecloth set over a deep bowl and let drip undisturbed for at least 2 hours or overnight. 

Using a kitchen scale, weigh elderberry juice, bottled lemon juice, and sugar.

Prepare boiling water canner. Heat jars in simmering water until ready to use, do not boil. Wash lids in warm soapy water and set bands aside. 

Combine elderberry juice, bottled lemon juice, and pectin in a large stainless-steel saucepan. Bring to a boil over medium-high heat, stirring to dissolve pectin. Add sugar, stirring until dissolved, and return mixture to a full rolling boil that cannot be stirred down. Boil hard for 1 minute, stirring constantly. Remove from heat. Skim off foam if necessary. 

Ladle hot jelly into a hot jar leaving ¼-inch headspace. Remove any air bubbles. Wipe jar rim. Center lid on jar and apply band, adjust to fingertip tight. Place jar on the rack in boiling-water canner with simmering water (180°F). Repeat until all jars are filled. 

Water must cover jars by 1 inch. Adjust heat to medium-high, cover canner, and bring water to a rolling boil. Process half-pint jars 10 minutes, adjusting for altitude. Turn off heat and remove cover. Let jars cool 5 minutes. Remove jars from canner; do not retighten bands if loose. Cool 12-24 hours. Check lids for seal, they should not flex when center is pressed