I still use DIY CO2 on my 29g, and have had to learn by doing.
I started with a single 2l coke bottle, airline tubing, check valve, and a Hagen ladder diffuser. I had a terrible time of getting CO2 levels up period, much less stable (which is as or even more important than the actual amount of CO2). I also have an inexpensive bubble counter in-line.
So I added a 2nd bottle and a brass T. That helped with making levels more consistent, but bottles just didn't keep producing long enough... I recently switched both bottles to 1 gallon juice jugs. I use the same recipe for the DIY "homebrew", just now they have more water so the yeast doesn't get overwhelmed by the alcohol as quickly- thus producing longer. It seems to be working very well.