Glad you asked...
There are a few different ways to ferment fruit juice depending on what you desire to make. One way is to simply let it age naturally and ferment in the process.
ddixon     How-To Expert
Then again, this answer might help you...
Fruit juice can be fermented using active yeast. You can find the yeast at your local grocery store. Simply, stir the yeast into the juice, let it set, and it will ferment.
paislie     How-To Expert
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