You don't need a fancy yogurt maker or special secret cultures to make yogurt at home cheaply and safely in your slow cooker -- just an accurate thermometer and some patience.
Wash all the tools you're going to use. Boil a large pot of water and submerge your equipment and jars to disinfect it. (The only bacteria you want to grow are the tasty kinds that make yogurt.) Remove the tools from the water and set them on a clean surface to dry.
Heat the milk in double boiler; you can just put it in a bowl or pot inside a larger pot of boiling water. Bring the milk up to 185 - 190 degrees F., until it begins to get frothy at the edges, but do not boil it. Check the temperature with a thermometer to ensure accuracy.
Remove the milk from the heat and start cooling it to 125 F. You can simply wait for it to cool, or place the pot with the milk in a sink of cool water and stir it to speed up the process. Again, check the temperature with a thermometer.
Pour the leftover water from the double boiler into the slow cooker; set the heat to 125 if you have a temperature control, otherwise set the heat to "Low." Add cool water or ice cubes until you reach the desired temperature, checking it with the thermometer. Any warmer than 130 F. will kill the bacteria, and cold water will stop it from growing so you to maintain a water bath of 110 - 125 F. If you are unsure about your thermometer, err on the cool side -- it will just take a little longer.
Split the milk between the containers (or jars) and add two good tablespoons of yogurt to each. Stir and cover each container tightly.
Place the closed jars with the yogurt-milk mixture in the warm water in your slow cooker and cover to preserve the heat. At this point you can turn it off and ignore it for about 8 hours, or leave the heat on "Low" if your slow cooker can maintain a steady temperature of 115 to 125 F. for about 3 hours.
Uncover the slow cooker, open the lid of one of the yogurt containers and tilt it to the side. If the yogurt is solid and a bit jiggly it's done; if it's runny it needs another hour or so. When the yogurt is done, remove it from the slow cooker and refrigerate.