Now mix the yeast, sugar and water solution as follows.
Put 1/4 to 1/2 teaspoon of yeast into the bottle with about a cup of warm water (bread yeast is fine). Some people have theorized that champaign or wine yeast should last longer (due to its ability to tolerate the alcohol generated from the brewing process), but recent tests reported on the net have not indicated any difference.
Shake to mix the yeast well.**
Add water to bring the solution up to 3/4 of the bottle.
Add 1 to 2 cups of sugar and shake well. The amount of yeast and sugar will determine the rate and duration of CO2 generation. More yeast will result in stronger CO2 production, but will exhaust the sugar quicker. Using 1/4 teaspoon of yeast and 2 cups of sugar will result in CO2 production for about 4 to 5 weeks.
In areas with soft water, some people recommend adding a teaspoon of baking soda to buffer the water and extend the life of the solution (prevent the acid formed by the brewing action from destroying the yeast prematurely).
The brew should generate about 1 bubble per second (from the air hose with no airstone), after about one day. Using warm water will help it get going faster. There is a ramping up period in the flow at the beginning and a ramping down period at the end of the solution's productive life. To smooth out the flow, you can use two smaller (1 liter) bottles, instead of one 2-liter bottle, and start one about a week or two after the other (but this doubles the maintenance effort). You should remove and clean the airstone occasionally, as yeast-generated CO2 has a tendency to generate a slime coating that gums up the airstone after a while.