Obtain the following:
1 - 2 liter bottle clean and empty
1 - Airline Check Valve
2 - cups sugar
6 - cups warm water (approximately 100 degrees. plus or minus 5 degrees)
1/4 - tsp bread or active dry yeast (red star, fleishcmanns, etc)
1 - ft length of silicon or rigid plastic tubing
X - length of same tubing as above long enough to go from bottle up and into tank.
1 - airstone - These come in different types wood, rock/stone, or synthetic micro bubbler. The synthetic works best and lasts longer exposed to the CO2. The rock/stone works better then the wood but will crumble apart after a few months.
Mix 1 cup of Warm water and yeast together in a non metalic cup until disolved. Allow to sit for about 5-10 minutes. If yeast was cold allow measured amount to warm to room temperature prior to placing in water or mix with 1/2 warm 1/2 cold water (cold first, slowly add warm to it) After waiting slight foam or bubble should be visible. If not do not be too concerned over this.
While waiting add sugar through funnel into bottle along with warm water, mix this well until sugar is disolved into water. Add yeast water mixture to sugar water and mix well. (Will have yeast particles floating in bottle...do not be concerned with this.. just mix back and forth 3-4 times)
Also needs various plumbers fittings or a small amount of aquarium silicone sealant. Do not use regular Silicone Caulking from hardware store. Must be aquarium approved sealant. Many silicone caulks have a anti-mold/mildew agent in them that can be toxic. Most hardware stores (home depot, lowes, etc) will carry this near the caulks in a special "glue" section for things such as goop and superglue.
If you do the fittings direction you should get some teflon tape as well. Use the fitting that are similar to the garden hose threads to fit on the top of the bottle. Do not use the hose type fitting though. Get the solid brass fittings that have no swivel piece in them. Solid metal with threads on both ends.. also get a hose washer. Insert hose washer into hose sized end of brass fittings. Connect various reduction fittings until you use the barbs to get the 1 ft tubing attached to the fittings. Attach other end of 1 ft tubing to check valve. Take the rest of the tubing and plug one end into the check valve. Attach other end to airstone. Do not insert in tank yet. Wrap top of soda bottle with teflon tape (about 10-12 times around), use pipe wrench or adjustable wrench on brass fitting. On flange that sticks out from 2 liter bottle attach channel lock type pliers or other coarse toothed plier like tool. Turn the brass fitting onto the bottle until it is very tight and you cannot keep hold with the channel locks onto the bottle. Now insert airstone into water. Run it next to or under the intake to your filter as this will difuse the gas into the water better.
After about 24-72 hours the airstone should start bubbling.
If you do not use plumbing fittings insert bottle cap into vice, clamp, and drill bottle cap. Use Aquarium Sealant to seal one end of 1ft tubing into bottle cap that has been drilled. Alternately you could also use Goop glue to seal this as well as Goop is one of the few glues that will stick to plastic used in soda bottle lids. Tubing should stick about 1/2 inch past the surface of the bottle cap. Seal well and allow to dry for at least 24 hours for aquarium silicone. Allow slightly longer for the goop glue as this will give off more fumes. Assemble rest of tubing as described for the plumbing method.
If you have further questions see the FAQ on my site or email me and I will be happy to assist.
During the first few days of running the DIY CO2 when it begins bubbling be sure to keep daily checks of pH until you are sure it is running ok. If you need to adjust pH due to changes CO2 has made do so with non phosphate containing pH adjusters. If you are in doubt ask on the message boards here. Many pH adjusters contain phosphates. This will cause an algae bloom with CO2. Some examples of those with phosphates is a product by TLC for pH down and pH higher. Most pH adjusters do contain this so you will have to be careful.
Additionally you will probably need to add Iron supplements such as Flourish Iron and/or use a Flourite substrate. Also consider ordering a fertilizer such as Tropica's Master Grow. Tropica is world recognized for quality plants and plant products (which plants are only sold as a franchise Licensed name in the US since Tropica's plants are not eligible for import here directly) most of all ask people if you have questions.
Lighting is always crucial too. If you supply CO2 but only have a "dinky little low power light" you're not going to see any results.