There are lots of instructions for homemade yogurt out there. I developed one that I think is somewhere in between too-casual-I-can-tell-this-might-not-work and too-scientific-I'm-not-buying-a-heating-pad. Find the recipe that feels ...
Wrap the slow cooker (lid on) in a thick towel and put it in the oven with the light on.
Boil some water in the tea kettle or pot and put that in the oven too. This keeps my oven at a perfect 100 degrees.
Let this sit undisturbed for 6 to 12 hours, depending on how tangy you like your yogurt. I do 8 hours.