Plain, sugar free yogurt may not be a big deal for you. But it is for me. Because it is hard to get here! The "plain" yogurt available in our town has sugar added to it for sweetness. Genuine plain yogurt is only available at the big grocery store half an hour from our house (and I've heard they only recently started stocking it), but I don't go there often. So, I've gone back to making my own which is something I haven't done regularly in a long time.
Here's how I make it. Be forewarned: this is more of a guide than a recipe. You may have to try it a few times until you get the consistency and flavor you are looking for.
1 scant half gallon of milk (Whole and 2% work better than skim. The idea behind the "scant" half gallon is that you want to have the right amount of milk to fill your containers when the milk becomes yogurt. I use two one-quart plastic yogurt containers but you could use one-quart jars or any other container that suits your fancy.)
1/2 cup plain yogurt with no sugar or fruit added to it (Use half of a small store-bought container to culture your milk. Choose a good quality one with lots of live and active cultures. You can use leftovers from this batch of yogurt to start your next batch, but the cultures will be weaker than in store-bought yogurt and after culturing from batch to batch a few times, the cultures won't work at all.)
1/3 to 1/2 cup dried milk powder
1. In a large covered pot on your stove, scald the milk (which means, bring it up to nearly boiling but don't let it come to a full boil because it like quadruples in volume and and turns into a monster that will end up all over your stove -- I have no idea how messy that would be, really, it's never happened to me).
2. Dissolve the dried milk powder in your hot milk.
3. Allow the milk to cool to about 115F. After 5-10 minutes of cooling, I speed the process along by filling my sink with room temperature water and putting the pot in the water. I keep the top on my pot so that the milk doesn't form a skin across the top.
4. Once your milk has cooled to about 115F, mix in your 1/2 cup of store-bought plain yogurt.
5. Pour the milk/yogurt mixture into your yogurt containers and keep them warm for 4-8 hours, depending on how firm and tart you want your yogurt to be. The longer you let it culture, the firmer and more tangy it will be. I let mine go for about six hours.
6. Once the yogurt is cultured and firm, put it in your fridge. It should stay fresh for about ten days.
Here are two ways I keep my yogurt warm while it cultures. At home, I used to wrap the yogurt containers in a heating pad on my countertop and keep the heating pad turned on during the culturing time. I don't have a heating pad here in Mexico (I sweat enough without one) so I discovered a new way that I think might be better:
Fill a few empty glass jars with boiling (or near-boiling) water and screw the tops onto the jars. Place them in a cooler along side your filled yogurt containers. With dry towels, fill in the extra spaces between and on top of your jars and close the cooler until you are happy with the firmness of your yogurt.
Once the yogurt is done, you can add to it whatever you want. Jam makes good fruity yogurt, maple syrup is tasty, plain yogurt is great for blending with frozen fruit and juice for a smoothie...but my favorite way to eat it is over cut up fruit and topped with something crunchy, usually nuts.
My yogurt-making skills came from my summer working at Rock-n-Water. Actually, I learned most of my cooking skills there. Amazing place, amazing people, incredible ministry.