We go through a lot of yogurt, about 2 quarts of plain greek yogurt a week. I started to get interested in making my own after running across a crockpot version. I also have a large surplus of milk because I get WIC but we don't drink milk often, so it was perfect. I mainly use this recipe, with just minor adjustments. I now make it at least once a week, it's really easy and delicious.
I use whatever milk I have on hand, usually 2% or 1%. The first time I made it with store bought starter, it seemed very watery, but I strained it until it was the texture I wanted, for several hours and it was fine, I just only ended up with about 3 cups. Once I used a half cup of reserved strained yogurt from that first watery batch for my next batch, it was much thicker. I don't know if using your own starter has more cultures, maybe because it was strained, or maybe I did something wrong the first time, but since then they have turned out great.
It it important to add the yogurt at the right temperature, if the milk is too hot it will not culture properly. I once accidentally added my yogurt when it was 180 degrees, and it never cultured, just looked like milk with some curdles. Whoops.
Crockpot Greek Yogurt
makes a bit over a quart (strained, but it can vary)
1/2 Gallon milk (any kind, but not ultra-pasterized)
1/2 cup plain yogurt (once you make it you can reserve a half cup for a starter)
sweeteners, vanilla bean paste or extract
Put the milk in a crockpot and with the lid on, set on low for about 2 1/2-3 hours, until temperature is 180 degrees (checked with a candy termometer).
Once at the right temperature, take the lid off and unplug crockpot and let cool until temperature is 95-115 degrees, about 1 hour.
Once at the right temperature, mix some of the warm milk with the reserved 1/2 cup yogurt to incorporate, and then mix into the milk. Put the lid back on, and place the whole crockpot into the oven with just the light on. Leave for 8-10 hours or overnight. The yogurt will be noticeably thickened at this point, with some watery whey also. It can look slightly lumpy, but straining will fix all that. Put the whole crock into the fridge for a few hours or into smaller containers.
Once cooled, line a mesh strainer with at least 2 layers of cheesecloth and place over a large bowl. Spoon the yogurt into the lined strainer and put back in the fridge for a few hours, to strain the whey. You can strain it until the desired texture, it will make it nice and creamy if you were concerned that it looked a bit lumpy at the beginning. I usually do it 3 hours, once I forgot about it overnight and it turned into yogurt cheese, but I just added back some of the whey and it was fine.
At this point you can sweeten it and flavor it. I will make some of it vanilla bean flavored. Using about 2 cups of yogurt I add 1/3 cup sugar (I suggest to add a small amount and taste until it is the desired sweetness) and whisk until dissolved, and add 1 teaspoon vanilla bean paste. You can also use the same amount of extract, but I highly recommend using paste or a vanilla bean.