- Nearly-boil a bunch of good whole milk. Let it cool down (or put the pan in a sink with some ice water if you want to get on with it) until it's 105 degrees or so. (Using a thermometer is the only part of this that requires supervision.)
- Mix in maybe a half-cup of existing plain yogurt. Put the milky mixture in some various small tupperwares.
- Find a heating pad or electric blanket. Put it in the bottom of the oven. Turn it on the lowest setting. Put the tupperwares on a rack above this. Close the oven.
- Leave. Let it sit for 6 or 8 or 10 hours -- until it seems firm enough to you. Yogurt!
Any yogurt can be used as the starter (incl. Activia), as long as it's plain and unsweetened. If you need honey, jam, etc., put it in after the yogurt is made.