--8 cups (half-gallon) of whole milk--pasteurized and homogenized is fine, but do NOT use ultra-pasteurized. (Debbie recommends starting with whole milk until you get the hang of yogurt-making)
--1/2 cup store-bought natural, live/active culture plain yogurt (you need to have a starter. Once you have made your own, you can use that as a starter)
--frozen/fresh fruit for flavoring
--thick bath towel
This takes a while. Make your yogurt on a weekend day when you are home to monitor.
I used a 4 quart crockpot. This is so exciting. My fingers are shaking!
Plug in your crockpot and turn to low. Add an entire half gallon of milk. Cover and cook on low for 2 1/2 hours.
Unplug your crockpot. Leave the cover on, and let it sit for 3 hours.
When 3 hours have passed, scoop out 2 cups of the warmish milk and put it in a bowl. Whisk in 1/2 cup of store-bought live/active culture yogurt. Then dump the bowl contents back into the crockpot. Stir to combine.
Put the lid back on your crockpot. Keep it unplugged, and wrap a heavy bath towel all the way around the crock for insulation.
Go to bed, or let it sit for 8 hours.
In the morning, the yogurt will have thickened---it's not as thick as store-bought yogurt, but has the consistency of low-fat plain yogurt.
Blend in batches with your favorite fruit. I did mango, strawberry, and blueberry. When you blend in the fruit, bubbles will form and might bother you. They aren't a big deal, and will settle eventually.
Chill in a plastic container(s) in the refrigerator. Your fresh yogurt will last 7-10 days. Save 1/2 cup as a starter to make a new batch.
Wowsers! This is awesome! I was completely astonished the next morning that the yogurt thickened. I was so excited to feel the drag on the spoon---and sort of scared the kids with my squealing.
They each ate a huge serving that morning (they added honey to their servings) and have eaten it for every meal for 2 days. I'm actually kind of worried they're over-doing it, but whatever. They're happy and are eating real food.
This is so much more cost-effective than the little things of yo-baby I was buying for them. I haven't run the numbers, because I sort of suck at math, but it's huge. Seriously huge.
Updated 10/23 8:45 pm:
I have gotten quite a few emails alerting me that yes, you can use lower-fat content milk with this method. To thicken the best, add one packet of unflavored gelatin to the mix after stirring in the yogurt with active cultures. Some have had good success mixing non-fat milk powder in as well.
The way I created fruit-flavored yogurt was by taking a cup or so of the plain and blending it in the stand blender (vitamix) with frozen fruit. Although this tastes great, the yogurt never thickened back up the way the plain did. I think maybe keeping the plain separate and adding fruit daily is your best bet. Or you can try the gelatin trick.
I was able to achieve a Greek-style yogurt this afternoon by lining a colander with a coffee liner and letting the liquid drip out of the leftover plain I made. The remaining yogurt was as thick as sour cream.