Wash the glass jars in warm soapy water and dry thoroughly.
Fill each jar with sweetened condensed milk leaving 1/2-inch head space. Twist the lid on securely.
Place the jars into the crock of the slow cooker leaving space between.
Fill the slow cooker with enough water to cover the jars.
Place the lid securely on the slow cooker and cook on low for 8-10 hours or until rich and golden brown in color.
Remove the jars carefully using tongs to rest on a dish towel. Be careful, they are hot.
Cool to room temperature then store chilled for up to one month.