In my previous grows, I use the CO2 during flowering... it really REALLy plumps up the bud.
Simple method is one cup oof sugar, one cup of water, and one packet of yeast. throw all that in a plastic container with a small (made with a nail or a screw) hole in the top. Just give the container a shake 15 minutes before the lights go out. One batch should last you a week or so. You can really stretch it out if you add a half-cup of sugar after a week and add a cup of water and a cup of sugar after 2 weeks. After 3 weeks of use, time to start from scratch...
There are better ways of getting CO2, but this is the cheapest.
Clearing Up Misconceptions About CO2 Augmentation.
-CO2 is heavier than air. It must be administered from above the plant.
-You need a sealed growspace.
-CO2 does no good at night/lights out.
-To be effective, you need a meter to guage CO2 levels. Atmospheric ppm meters cost $500.
-CO2, like nitrogen, helps primarily during vegatative growth (which includes the first 3 weeks of 12/12 i.e. the "stretch"). It does almost nothing during flowering.
Most indoor cultivators grow thier plants to a certain height before inducing flowering. CO2 augmentation helps during this phase. It shortens the time it takes to get the plant to that target height.
It does not increase bud size. If you had 2 identical plants in side by side growroom (like I did), one with a $1,000.00 CO2 system/the other with excellent flo-through ventilation, you'd end up with the same size buds. The one's with add'l CO2 would be finished about 5 days sooner.
For most home growers, excellent flo-through ventilation ($30) is more cost-effective than CO2 augmentation.