Turn on the light in oven and leave on. Measure and pour milk into a medium saucepan. When it is warm, add sugar, and vanilla extract. Gently stir. (you can taste it now to see if you like the flavor and adjust to your liking)

Heat on medium (7 mark on conventional stove) until just before boiling point. Stir frequently. If you have a candy thermometer, the temperature should reach to 175-180 degrees after you stir it. Turn off heat and remove from the stove and allow to cool. If a skin forms, remove it.

Allow milk to cool until temperature drops to between 110 and 120°F. This would feel like the temperature of a baby's warm bath water. (place a drop on the underside of your wrist to check temperature if you don't have a candy thermometer.) Another way to tell if it's cooled enough, is to put your hand on the side of the pot. If you can keep your hand there without it burning you, it's cool enough.

In a measuring cup or small bowl mix a bit of the cooled milk and the starter yogurt together. Add to the rest of the milk and stir well. You may use yogurt from a previous batch of your own homemade yogurt.

Pour milk into a large glass or ceramic bowl or container. Cover container with a clean hand towel and place in the warm oven and leave the light on.

Allow to incubate for 10 hours. (DO NOT OPEN OVEN DOOR DURING THIS TIME)

After the 10 hours, chill the yogurt. The longer you leave it in the oven will result in the yogurt becoming more tart. It is your choice to strain off the whey or stir it back in. Straining makes the yogurt thicker and less tart resulting in Greek yogurt.

To strain, very carefully pour yogurt into a cheesecloth-lined strainer which has been placed over a deep bowl.

Empty whey from bowl and pour yogurt out of strainer back into the original bowl. Use a good whisk to beat until smooth. Add milk if yogurt is too thick.