In a 4-quart saucepan whisk together milk.
Cook over medium heat until an instant-read thermometer inserted in milk mixture registers 180 degrees F to 185 degrees F, stirring frequently.
Remove from heat.
Cool at room temperature until thermometer registers 110 degrees F to 113 degrees F, stirring thoroughly before checking temperature. (The milk can be cooled more rapidly by placing the saucepan in ice water for a few minutes.)
In a small bowl stir together 1 cup of the warm milk mixture and the starter until smooth.
While stirring constantly, slowly pour the milk-starter mixture back into the milk mixture in saucepan.
Ladle mixture into four clean pint canning jars, leaving a 1-inch headspace.
Adjust lids and screw bands.
Place filled jars in a 6-quart slow cooker.
Pour enough lukewarm water (about 108 degrees F) into the cooker to reach just over halfway up sides of jars.
Cover cooker and cook on high-heat setting for 5 minutes.
Turn off cooker, wrap with a thick towel, and let stand about 4 hours or until yogurt is thick, turning on cooker to high-heat setting for 5 minutes every hour. (Use a thermometer that can be attached to the cooker so you can monitor the temperature)
Remove jars from cooker.
Chill about 4 hours or until yogurt is set.
Store, tightly covered, in the refrigerator for up to 2 weeks.
If desired, reserve some of the yogurt for making a future batch.
Note: For Greek yogurt, drain yogurt overnight through a strainer lined with cheesecloth.