Article - Make Soda Yourself With Yeast, Sugar & Bottles

[TWP]

Very simple process and it can be self-sustaining so you don't need to keep buying more yeast.

http://modernsurvivalblog.com/survival-kitchen/make-soda/

Do read the warnings on preparation and temperature.

Also note that the carbonation will rapidly dissipate.  Commercial soda uses phosphoric acid to lower the Ph of the solution which will keep the CO2 from quickly escaping.  The advice on smaller bottles (one serving size) will make this less of a problem.  It is unlikely that you will have phosphoric acid, nor even want to use it because it has some health hazards relating to calcium loss from your bones... FYI

[230gr]

For homemade beer, I use 2½ tsp sugar per 1 liter plastic bottle for carbonation. I like the plastic containers because you can feel how taut they are from the carbonation pressure.
   

[TWP]

What are you using for the ferment?  natural yeasts or something else?

[230gr]

natural yeasts or something else?

just the natural yeast that is in the beer.  There is still plenty left in the brewed beer just waiting for more sugar of some kind. The original sodas, birch beer, root beer, where really beer and slightly alcoholic (or more so if you fed it more sugar to start with).