What is it?...it is smelly, messy and IMHO not worth the effort. For co2 to be enhanced effeciently, you really need a sealed grow, with thermostaically controlled exhaust/intakes, and meters, guages 'n tanks.
That being said, I think the recipe is 2 cups of sugar, 1 package of yeast in a gallon of water. Remember, co2 is heavy, so it should be dispersed above your plants.