In the past when I made yogurt, I just used about 1/4 to 1/2 cup of my last batch for the starter to make the next one. I would set my prepared jars (I used old jam jars) of mixture into a kettle of very hot, but not boiling, water (deep enough to come up to the neck of the jars, but not seep into the jars) and wrapped the whole thing in two nice thick towels. In 4 or 5 hours I had very nice yogurt.