This is not a new concept, or my own concept. But it's one I love and have been faithful to for years, and a recipe I swear by.
Originally, I found the recipe here. Another great easy-to-follow rendition is here. After giving you those links I know I don't have much else to say on the subject but I'll give you my "plan of attack" anyway.
In the crock pot goes 8 cups of milk. I use whole milk because that's what I buy. You can use 1% or 2%. Turn the crockpot on low and let it sit for 2.5 hours.
After 2.5 hours turn the crock pot off, unplug, and let it sit for 3 hours. Now, you can let it sit longer if you want (or if you forget about it ... not that I've EVER done that. much.) but it leads to more sour yogurt if it sits too long. Just know that you don't have to be rigid with any of the times, but they do seem to produce the best results.
After 3 hours add 1/2-1 cup yogurt (I use one single serving container). You can use plain or flavored, regular or greek or one of the "plus" kinds ... the greek and plus ones seem to produce slightly thicker yogurt and the other cultures in them also seem to grow (I don't know this for a fact, but I choose to believe it), so I go with these. I add 1/2-1 cup sugar or honey at this time as well, stirring until it's dissolved. Sometimes, if I have it on hand, I add in a tbsp. or so of vanilla, but if I don't have any I don't sweat it.
Cover the crock pot with a thick towel or blanket and let it sit for 8-12 hours.
The yogurt will generally be slightly runnier than regular yogurt. That's fine. For babies I freeze little bits in ice cube trays and give them that to work on - great for teething and lets them eat it themselves.
I store it in the fridge in ziploc containers (containers, not bags). If I have fruit on hand I'll add it in before storing so the kids can just grab and eat. You could also freeze the yogurt, in this house I'm lucky to not run out even when making batches back to back.
Or I make smoothies with yogurt, ice, raw spinach, and fruit ... and whatever else we have on hand. The spinach will make the smoothie green but it won't make it taste like spinach so it's a good way to up that iron and green leaf intake. I freeze little bits of that for the babies too.
It didn't take long at all for my kids to decide that they liked Mama's yogurt better than store bought.
1/2 gallon of milk is about $1.50, 1 cup yogurt is about $.50 (not that I'd ever actually pay full price for it, but if I did it'd cost that) ... total cost for 9 cups of yogurt = $2 ... about $.22 a cup. Single serve yogurts at the grocery store are less than 1 cup, so this ends up being a really good price, it doesn't beat a super Publix BOGO deal topped with coupons (which is when I stock up on the starter containers of yogurt and freeze them for future use), but still, it's a really good price and you have more control over the final product.