First you should use good probiotics. Just put one of those expensive capsules in a cup of a full fat fresh milk in the morning. I don't boil, but if you want to kill all bacteria in pasteurized milk, boil it. Leave it in a room temperature (normally 27C) for 10-12 hours. Then store in the fridge overnight. My yogurt turns out perfect like silky and delicate tofu. Right out of fridge, it's not sour.