I used water kefir to ferment the herbs and spices. It doesn't quite have the flavor of root beer that I remember from A&W as a kid, which is artificially flavored. It is the real deal. I found that mint, birch bark, sarsaparilla, and vanilla give it that root beer flavor. There are some good recipes out there. I'm still perfecting it but it came out good enough to drink. You need at least a cup of dissolved sugar to get 1/2 gallon, I'd say. I don't drink too much at a time because the herbs can be medicinal. I enjoy a small shot at a time, or add some to an electrolyte beverage for dinner to aid digestion. A little can go a long way. Along with root beer, I have plenty of ginger ale on hand, plus beet kvass and pickled egg juice. I have many different fermented beverages to choose from.
Following Weston A. Price