yogurt maker flavorings

I have a new yogurt maker and I don't know how to add flavorings. The book that came wih it does not tell me anything but how to use it in two languages. I have figured out how to add sugar and vanilla flavorings after throwing one batch of yogurt away. Can somebody tell me how to add the flavoring and what do I add? Can I add Lemon juice to make lemon flavored. I am slowly getting this figured out.