This is a pretty basic diagram of a DIY CO2 setup. You can choose to have 1 bottle, or two. The smaller bottle is a "bubble counter" or "separator", that you fill with water above the input line, and out the line to the tank, which stays above the water line. This allows you to monitor true flow outside the tank, and if any yeast mixture leaks through the tubing, it is deposited here, instead of in your tank. In the tank, you can use your choice of diffuser, whether it is a chopstick as mentioned above, a limewood airstone, or a reactor setup. the little nubby part (labeled "3") is a backflow preventer, or checkvalve, which prevents your tank water from siphoning back into the bottle. that is a must.