I heat my quart canning jars and some kitchen towels in a warm oven while I'm heating the milk.
Then after filling the jars with the yogurt starter/milk mixture, I wrap them with the warm towels and put them into one of those hot/cold bags that you can get at your grocer's. (The kind that says it keeps food hot/cold for up to 3 hours.) I put one warm towel on the bottom of the bag, one around each quart jar, and one on top of the jars before I seal up the bag. ETA: The hot/cold bag will hold 4 quart jars.
Then I leave it on my kitchen counter for about 6 hours. Sometimes, I leave it over night. Most recipes will tell you not to let it sit that long, because the yogurt can become too tart. Since the majority of our homemade yogurt gets fed to the chickens, I'm not worried about how tart it gets.