KP, I put about 5 cups of water in a milk jug, 2 cups of sugar, and one packet of yeast. Then I put a hole in the milk jug and put a tube in the hole, um, it's about an inch around. The jug sits on a shelf above my plants, with the tube pointing down, since CO2 is heavier than air. I found the "recipe" on the net somewhere.

It works, but sometimes it gets messy if the mixture gets real frisky and bubbly. And when my temps drop below 72F, the yeast dies.