Yep, it's super easy.
Get these supplies:
A two liter bottle with cap
Some air tubing
2 cups sugar
1/4 teaspoon active dry yeast
an air tube connector
(
http://www.thatpetplace.com/images/p8813B.JPG)
optional: a wooden air diffuser
1. rinse the 2 liter out good
2. toss in the yeast and sugar
3. fill with warm water. it must be warm to activate the yeast, but it should not be hot. just barely warm. I think the max temp for yeast is 110 degrees - so barely warm at all. Fill the 2 liter all the way up to the point where it starts to taper in. you need to leave some breathign room.
4. drill a little hole in the cap of the bottle. take care to make it circular, not all rough and shoddy, and just wide enough to fit the air tubing. i use a steak knife to make the hole. nothing fancy
5. push the air tubing into and through the cap.
6. stick the air tube connector to the bottom of the air hose. so that it's on the "inside" side of the cap.
7. pull/push the little connector back up snugly into the hole you drilled in the cap. the air connector makes the air tubing wider, so when you force it into the hole it gets all snug and airtight. no silicone required!
8. put cap on, situate the co2 bottle somewhere safe near tank
9. place the outlet of the air tube in the aquarium. i lash mine to the filter intake so that the filter handles diffusing the co2 in the water for me. if you don't want to do that you can just use a good air diffuser and place that in the aquarium. use a wood one, they make "fine" bubbles.
hope that helps!
good luck!