I used to watch my grandmother make tvorog when I was young. She would heat milk to lukewarm, add buttermilk, turn off the heat and let it ferment for a day or two depending on ambient temperature, so it's just a soft lactic cheese, much like ricotta in texture, but with lactic cheese flavor. She never used rennet. Some of her friends used yogurt for the culture instead of buttermilk. They never used rennet either. One of them used to heat the curd to very warm but not boiling once the curd was set, then drained it. It made a firmer and drier tvorog than my grandmother's, more of a cottage cheese-like texture. Each of them claimed they made real authentic Russian tvorog
.
I have seen tvorog on sale in San Francisco under Pavel's brand and to me, it just seems to be a more expensive ricotta.
Like any ethnic food product, there are a thousand different versions, many of them very similar to each other. I would play around with it and work out a version that you like and that works in your cooking and then it will be your unique tvorog. If you want to use rennet, I would probably use a drop or two per gallon of milk. I really liked my grandmother's tvorog because it had a buttery flavor to it, so like mikekchar said, Flora Danica would be a good culture to try, or just use a buttermilk that you especially like. Good luck!
