Techniques for making yogurt if you do not have an appliance designed for it.
With a thermos
Almost fill a thermos bottle (preferably widemouthed) with milk heated to 100 degrees F. Add 2 tablespoons of plain yogurt and mix thoroughly. Put the lid on and wrap the thermos in two or three terry towels. Set it in a warm, draft-free place overnight.
In an oven
Pour 1 quart of milk into a casserole dish and add 3 tablespoons of plain yogurt. Stir well and cover the casserole. Place in a warm (100 degree F.) oven with the heat off. Let it sit overnight.
On a heating pad
Mix 1 quart of milk and 3 tablespoons of plain yogurt. Set an electric heating pad at medium temperature and place in the bottom of a cardboard box with a lid. (A large shoebox works well.) Fill small plastic containers with the milk-yogurt mixture; put on the lids. Wrap a heating pad around the containers, then cover with towels to fill the box and let sit, undisturbed, for 5 to 6 hours.
In the sun
Pour 1 quart warmed milk into a glass-lidded bowl or casserole. Add 3 tablespoons plain yogurt and cover with the glass lid or a clear glass pie pan. Place in the sun on a warm (not too hot) summer day and let sit 4 to 5 hours. Watch it to make sure it is not shaded as the sun moves.
On the back of a wood-stove
Many grandmothers made clabber by setting a bowl of freshly drawn milk on the back of the stove after supper. Make yogurt this way by adding 1 cup starter to 2 quarts milk and let it sit, loosely covered with a dish towel, on the back of the cooling wood range overnight.
In a Crock-Pot
Preheat a Crock-Pot on low for about 15 minutes, until it feels very warm to the fingertips. Put covered containers of yogurt mixture into the Crock-Pot, cover it, and turn off the heat. At 35- to 45-minutes intervals, heat the Crock-Pot on low for 10 to 15 minutes.