With a fork, pick elderberries off their stems. You can freeze them beforehand to make this easier.
Place the elderberries, lemon juice, apple juice concentrate, and sugar in a medium pot and bring to a boil over medium heat until the elderberries are mostly popped.
Add the pectin, stir well, and continue to boil for another 5 mins.
Do a gel test: drop a bit of jam on a cold plate. If it gels, it's ready. Otherwise continue boiling it or adding a bit more pectin.
Fill in sterilized jars, leaving about 1/4 inch to the top of the jar. Put lids on a process as desired, such as pressure canning or water bath canning.