Grab a few handfuls of grapes, give them a rinse in water and remove from the stems. Slice the grapes in half and put them in a large jar.
To the sliced grapes in the jar, add equal parts unbleached white flour and room temperature water. Give it a good stir with a wood spoon. Then cover the container and allow to sit for three days at room temperature.
After three days, pluck out the grapes using a wood spoon. Discard half the mixture.
Then feed the remainder left in the jar with equal parts water and flour (1 C flour and 1 C water). Stir well, until the mixture is thoroughly combined, then cover the container.
Let that starter sit for another 24 hours and it should be ready to use.