Do some reading on the web, google for "make yogurt" and stuff like that. You want to heat the milk to just under the boiling point and let it cool. Always be very clean, and add the (live) yogurt to this milk. It wants to be kept at 110F, hotter and you may kill the bacteria and lower and you'll have conditions better for pathogenic bacteria to grow. With some experience you'll be able to tell easily if it's bad by smell and texture. It shouldn't be slimy. It only needs about 8 hours to fully culture but if you leave it for 24-36 hours at proper temperature it'll convert more (all?) the lactose to acids. Lactose is milk Sugar that is hard to digest, fully cultured yogurt like this is very good for candida and very easy to digest (commercial yogurt isn't because they culture it as quickly as possible). Try eating it fresh and warm as soon as it's done, the bacteria is so strong and healthy it often makes my lips tingle. This is also the best way I've found to recolonize the colon, hold a fresh yogurt enema for a few hours and you'll notice the change.