Boil the potato slices in the cup of water until they are soft. Mash them in the water then set aside to cool for about 10 minutes. Stir in the sugar and flour. Cover and leave in a warm place to provide. (You can use this in your recipe as soon as it becomes bubbly)
To make more rewena, reserve 1 tablespoon of the provided mix and put into a jar. Feed it with 1/2 cup warm water. Feed it the next day with 1 teaspoon of sugar. Continue to feed it, water one day, sugar the next.