I'm assuming you don't keg. I do root beer with Zatarains extract, water, and Splenda. Cheaper than diet coke, without nutra-sweet poison.
If you use yeast to ferment do NOT use glass bottles. It is dangerous. Only use plastic. (If you goof ald let the ferment go to long, exploding glass can be bad for your skin, eyes, etc).
I'd follow the printed recipe that comes with the extract.