Do you mean CO2 generarator or diffuser? On the basic level, both are very simple.
Generator:
Drill a hole in a bottle cap slightly smaller than your airline tubing, cut the tubing on an angle and use pliers to pull it through. (This works best with silicone tubing.) Put the cap on the bottle. Ta-da, you have a CO2 generator. Fill the generator with a cup or two of sugar, a little yeast, and alot of water. (I also put some gravel in mine to keep them from tipping over easily.) There are lots of diffrent recipies online, pick one and stick to it, and you'll get a feel for how often you need to recharge the generator.
Diffuser:
The simplest thing is stick the tubing from your generator into your filter inlet. The impeller will break up the bubbles, and the bubbles will get caught on the filter media and have a long time to disolve before rising to the surface. Next easiest is a fine pore airstone. After that you can get fancy with methods of trapping the CO2 underwater, or slowing it's ascention, or circulating water over it.