It is a by-product of making wine from grapes. Grappa is made from not very well pressed grape marc and seeds. Adding grape juice is a mistake. The distillate is then less aromatic. The liquid must be properly acidified, the yeast mother, enzymes and yeast medium must be added. Mix thoroughly and whisk the liquid well to squeeze the air. Tighten the fermenter cover with the fermentation tube. Distill preferably in a double-walled tank with stirrer. If you don't have one, pour water after the fermentation and draining is complete and then squeeze well. After double distillation in alembic, the distillate must be poured into a barrel, and after spilling and final dilution, it should be left for a long time.
You have to beware of grapes that were intended for trade, as they are usually preserved, which may prevent fermentation.