Yeast is an essential ingredient in making cider. Yeast converts the sugar in the apple juice into alcohol and carbon dioxide. For best results, use fresh yeast. To add yeast, mix 1/2 cup of warm water with 1/4 teaspoon of sugar and stir into the cider. Wait 10 minutes, then add the yeast. Stir again and let sit for 3 to 5 hours at room temperature, or until the yeast is bubbly.