I am in love with the Food52 featured recipe by Alana Chernila. It's perfect.
I follow the 'put it in a warm place' method. I start with 8-10 cups of the best, least pasteurized milk I can find; locally that's Kilgus Farms. Store bought won't work as well, or frankly, at all, so don't waste your time.
Heat the milk on low-medium, stirring frequently until it reaches 185 degrees (be sure to use a thermometer!) Let it cool to about 110. Stir in 1-1 1/2 cups of your favorite plain yogurt. I usually pick on that has the most science-y sounding bacteria listed.
At this point you can transfer it to a crock pot and set it to warm OR put it in your over, with a lid, on the 'proof' setting if your oven has that. Mine does and it's one of my favorite things about our house.
Let it sit. DO NOT BOTHER IT. Don't just 'take a peek' or 'jiggle it to see if it's set.' Seriously. Leave it. I often start my whole process around 6 pm and then put it in the oven overnight. Even though the original post says 5-7 I have much better luck with more like 8-12 hours.
Take it out and strain it at this point if you wish. I usually do to get more of a Greek yogurt like consistency. To do this I put a large colander over a larger bowl, then line the colander with a thin dishtowel or cloth. Dump in the yogurt and let it sit for about 20 minutes. It really doesn't take very long and you can always let it go a bit longer if needed.
After this, refrigerate and enjoy! If it's lumpy it may be because the milk scalded. To remedy just stick it in a blender or use it for smoothies. Be sure to save a 1-1 1/2 cups for your next batch! Good luck and let me know what your favorite uses are for this yogurt!