I've had a lot of success with plain Kirkland greek yogurt but most should work. I also do 1 gallon at a time and microwave it to get it up to temperature. This takes about 20min from fridge to desired temperature. After fermenting in my instant pot, I pour it into a fine mesh strainer on top of a large food container and put it in the fridge. The container is for the whey to collect in. This turns it into Greek yogurt.