I hope this make sense. I tried to add in what I do. Good luck and enjoy. I am not an expert but If you have any questions let me know and I will try and answer them.
* 1/2 gallon whole milk (can use whole, 2%, 1% or skim milk- the higher fat the creamier and thicker the yogurt)
* 1/2 cup room temperature plain yogurt with "live cultures" (I recommend Dannon Plain Yogurt, because it is made with milk and cultures, other brands have other fillers in them and I don't want that.)
1. Bring milk to just a boil and then set aside to cool. While you are heating the milk remember to stir almost constantly so it doesn't burn on the bottom. (I have forgotten to stir frequently and it burned, but I still able to use it. If this happens just don't scrape the bottom or your yogurt will have cottage cheese like curds in it.)
2. Cool just enough not to bite the finger to touch, (about 120°F). (I don't have a thermometer so the trick is to let the milk cool until you can keep your finger in it to the count of ten. It takes about 45 minutes to get to this temperature.) Sometimes a skin will form while the milk is cooling. I like to remove it because it feels like plastic in your mouth when eaten. But it doesn't have to be removed.
3. Pour warm milk in a glass or Pyrex bowl and add the plain yogurt with"live culture".
4. Mix well by stirring the starter in, and cover.
5. Completely cover the bowl with towels top and bottom to maintain an even temperature. I put mine into a cooler and surround it with towels to keep the temperature warm.
6. Keep covered at room temperature until the cultures have set, about 6-8 hours. The longer it sits the tangier it gets.
7. Carefully pour off the whey (liquid) that forms on the top.
8. Refrigerate for 4 hours before serving.
9. To store, keep in refrigerator.
10. If you like, you can add a spoonful or two of fruit preserves/jam and a bit of vanilla before serving.
This will make a runnier yogurt. If you would like to make it thicker follow these instructions:
Follow instructions above until #7 then do this:
1. Line a medium-large bowl with a piece of cheesecloth or a clean white dish towel.
2. Dump a the yogurt into the center of the cloth.
3. Bring the four corners of the cloth together and lift the yogurt.
4. Place the cloth containing the yogurt in a strainer or colander, and place the strainer or colander in a bowl where it doesn't touch the bottom (so that the liquid can continue to drain).
5. Place the bowl containing the strainer/colander in the refrigerator and allow to drain for 2-3 hours. I dump the whey out of the bowl every hour. Strain until you get the consistency you like.
6. After straining using a spatula, put the yogurt in a bowl for use.
7. Return yogurt to the fridge another hour or two.