Mix together the canned milks and water. Heat over low heat to temperature of 105 degrees. This is like warm bath water, if you don't have a thermometer. Remove from heat and stir in the yogurt until well dissolved.
Pour yogurt mixture into clean, warm, wide-mouth pint jars. Cover with clean lids. Place jars in a place where they will remain at 105-110 degrees for several hours. Here's what I do:
Put a large plastic cooler in your sink or near a heat vent in the winter. Put the jars in it, and fill it with hot water (hot from the tap) to about half- or two-thirds of the way up the side of the jars. Press a plastic bag down around jars to seal out air, and cover with several towels and then the lid to the cooler. The purpose of the plastic bag is to keep the towels dry and the purpose of the towels is to keep the heat in as long as possible. I used to just seal my towels inside a garbage bag and set it on top of the jars.
Check every hour for 5-6 hours to be sure water is still very hot. Remove some and add boiling water to keep it hot. Water should not be boiling, but should be like hot water from the tap and not drop too much below 110 degrees.
Also look at the yogurt after a few hours, tilting the jars to see if it has set up yet. Once yogurt has thickend and set up, you can put it in the refrigerator and use as regular yogurt. This should take about 6 hours, but may take longer depending on the culture in your starter. Be sure to save one cup of your yogurt to make your next batch, that way you don't have to buy more.
I have also done this in the oven - set the jars on a cookie sheet and set the oven to warm. Keep an oven thermometer in there and be sure the temperature does not go too far above 110 degrees, for 5-6 hours. Remember, time is approximate and yogurt may set up sooner or later than the 5-6 hours. One problem with using the oven is that it ties up your oven for several hours. Another option would be to use a warming drawer if you have one. Just be sure to use a thermometer to keep it at 110 degrees, or the excess heat will kill the culture.
I've also heard you can process your yogurt in a crock pot, but I've never tried this. I found 2 methods. One where you put the yogurt directly in the crock and turn it to low for 3 hours and then shut the crock pot off for 7 hours. The other where you put the yogurt into jars and set them in hot water in the crock pot on low for 2 hours, then turn off for 2 hours, then back on for 2 hours. If using this method, I would definitely use a thermometer to be sure the water didn't get too hot. Not sure how many jars you could fit into a crock pot.