I started from this recipe http://www.homemademommy.net/2013/08/how-to-make-beet-kvass-that-actually-tastes-good.html
But I changed it so much that I will write down my own.
A Quart Jar of Beet Kvass:
Cut up a small wedge of cabbage into one to two inch chunks, enough to fill the bottom couple of inches of a quart jar
Add a couple of small slices of ginger
Add two medium sized beets (scrubbed in cold water but not peeled or cooked) cut into 1 to 2 inch chunks, enough to fill the jar 1/2 to 2/3 full
Add 2 teaspoons of natural salt. I use fine ground Redmond Real Salt, which is reasonably priced and not purified
Fill with well, filtered or spring water to 1 inch from the top of the jar.
Cover (see notes below) and let sit at cool room temperature 3-5 days, then strain and drink the liquid. The beets can be used a second time (I filled the jar back up with water and added 1 teaspoon of salt) or cooked.
The salt and the acid produced by the fermentation mean you don’t have to worry about botulism. The way this kind of ferment will go bad is that it will grow mold, and if that happens it is safest to throw the whole thing out. Things that help prevent mold:
• Make sure there aren’t little bits of cabbage or anything else floating on the surface (keep your cabbage pieces large and put the beets on top to hold them down)
• If you have problems with mold you can increase the amount of salt to a tablespoon, particularly if you are using kosher salt. If you want to use less than 2 teaspoons of salt try a tablespoon of raw honey and one teaspoon of salt, but be aware that a sweeter ferment produces more alcohol.
• Use a lid that lets the gasses produced by fermentation out and doesn’t let oxygen in. My current favorite is: http://www.masontops.com/products/pickle-pipe. You can also get plastic mason jar tops drilled for a beermakers airlock or just use a two part metal lid and screw it down firmly but not all the way tight (or a clamp style lid that isn’t extremely tight).
• Put the jar in a dark place like a cabinet. The ideal temperature is in the low to mid 60s, but warmer or cooler temperatures will mostly affect the speed at which it ferments—taste it and see when it is to your liking
• A thin whitish layer on the surface, if it is not fuzzy, is ok. That is kvam yeast.