Boil water for five minutes with whisk in it. This sterilizes everything that will touch the yogurt.
Turn your oven light on (this should be enough to keep it at incubating temperature). If you don't have an oven light, you can wrap the pot in towels and keep it in a warm place.
Dump the water out and add the bag of milk and 3 Tbsps of instant skim milk powder to the pot. Stir.
Whisk frequently on medium, making sure to scrape edges and bottom of pot, until there are tiny bubbles like latte foam and there is a significant amount of steam. DO NOT LET IT BOIL!
Place pot in a cold water bath. Wait until milk is lukewarm.
Only once the milk is slightly warmer than room temperature, add your plain yogurt starter. Mix well into milk.
Place the pot in the oven (keeping the light on) and set a timer for 6.5 hours.
When you pull it out, it might still seem somewhat liquidy, but this is just because of the warmth. It will get thicker in the fridge.
Separate into two yogurt containers (that have been washed, of course) and put them in the fridge!
Milk fat %, amount of milk powder, and incubation time are what affect the taste and consistency of your yogurt. More of each makes the yogurt thicker, but more time makes the yogurt more sour.
I don't like my yogurt very sour so I add the milk powder so that I don't have to leave it in the oven as long. Feel free to play around with it though!
Finally, I go through two containers of yogurt in 7-10 days (it becomes very addictive), but if you eat less, consider halfing this recipe. Don't forget to leave a little bit to start your next batch!