From SeriousEats.com

DIY Homemade Greek Yogurt

Note: Both the milk and the starter you select will impact the taste and consistency of the final product, and you will likely need to experiment some before you hit on your ideal ingredients. If you’re using fresh yogurt as your starter rather than freeze-dried, be sure to use a plain variety that contains live and active cultures.

There is more than one way to incubate at 110°F—from a simple thermos to a special appliance specific to yogurt making. I found the method below to be both simple and consistent, but you may want to experiment and use the process you find most efficient.

A useful trick I picked up from Alana Chernila’s The Homemade Pantry is to incubate a small portion of the milk and starter mixture in a 1/2-pint jar alongside the two filled quart jars. Reserve this smaller portion to culture your next batch of yogurt. If using the small cooler method outlined below, you can keep this smaller jar above the water line by setting it on top of a narrow, empty 1/2-pint jar.

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About This Recipe

Yield:	makes 6 cups of strained Greek-style yogurt
Active time:	1 hour
Total time:	15 hours
Special equipment:	two quart-sized jars with lids, one 1/2-pint-sized jar with lid, one small cooler, cheese cloth, thermometer
This recipe appears in:	DIY: How to Make Greek Yogurt at Home

Ingredients

8 cups milk
4 tablespoons plain yogurt with live and active cultures or freeze-dried yogurt starter

Procedures

In a heavy-bottomed pot over medium heat, bring milk to 180°F, stirring regularly to prevent scorching. Once milk has reached temperature, allow it to cool to 110°F (place pot in an ice bath to speed cooling, if desired).
When milk has cooled, add yogurt to the pot and whisk thoroughly to combine.
Pour milk and starter mixture into two quart-sized jars (and smaller 1/2-pint, if using) and screw on lids. Place them in a small insulated cooler and fill with 120°F water until jars are submerged nearly up to their lids. Close cooler and leave in a draft-free, undisturbed place for six hours or until desired tartness is achieved.
When incubation is complete, remove jars from water bath and place in refrigerator for at least six hours to halt culturing and set yogurt.
At this point, yogurt may be eaten, but to achieve a Greek-style consistency, it will need to be strained. Place a fine mesh strainer over a bowl and line it with two layers of cheese cloth. Spoon yogurt into lined strainer and allow to drain for two hours or until desired thickness is achieved.
Transfer yogurt to a storage container and refrigerate until needed. Remaining leftover whey (approximately two cups) may be reserved for another use if desired.