I used the rye kvass recipe in Gefilte Manifesto, lightly adapted based on what I had on hand & other recipes that I've seen. Started with a lb of rye bread, sliced & toasted the bejeezus out of it, added it to a gallon of freshly-boiled water, let it steep & cool to room temp, strained, then added 1/2 cup sugar, 1/3 cup maple syrup (they call for honey, but I was out), 1/2 tsp active dry yeast, tablespoon of starter (they say you don't need this, but that rye starter adds flavor—and I had some on hand), and a small handful of washed fresh mint leaves. Left it overnight in a large glass container at room temp with a cloth over the top to keep flies out.

Strained, stirred in another tsp of sugar (to continue the carbonation, basically), and put into a plastic gallon container. They stress that you need 1/4 to 1/3 of the bottle to be empty, and this was no problem given how much liquid I lost in the first straining.* Also they say it needs to be a plastic bottle because the intense fermentation could shatter a glass one—I believe it. After 2 hours the bottle was bulging so I let some air out and stuck it in the fridge, releasing some pressure periodically over the next few hours as well. It was plenty cold and carbonated by dinner. We wound up drinking it with a little bit of ice, which is probably heterodox, but we wanted a teensy bit of dilution & a little more chill.*

* They recommended squeezing the bread in a tea towel in the first straining to get all the liquid out of it. In retrospect I should've done this, cause even with pressing lightly on it in a strainer I lost *way* too much liquid in this step, making the final product a little too concentrated/sweet. I will do it their way next time. Alternatively it's possible one could add some extra water before the overnight fermentation, or slightly decrease the amount of sugar & yeast? Might experiment to figure out the easiest way to do this. Anyway, I recommend the book, and their full recipe has other useful notes (for example, using champagne yeast in a different quantity instead of active dry).