Pick grapes off branch. Pulse them in a food processor to break skins and partially extract juices. Put everything (juice, skins, seeds) in a non-reactive container and allow to ferment (the juice will become less sweet as the sugar turns to alcohol). Ferment until juice starts to become a bit effervescent. Process in small batches in a blender to extract juice. Do not process for a long time, as agitating the seeds will make the juice bitter. Strain through fine-mesh strainer, pressing down with your hands to release as much juice as possible. Freeze juice and scrape several times during the freezing process to produce a fine textured granita.