I have done this by using a slice of real sourdough bread or real pumpernickel bread. Regular store bread won’t work. Use a pea size speck of your blue mold and let it set on the bread in a moist container. Once fully covered in blue mold then air dry the bread and either zip lock or vacuum package. It will keep indefinitely if completely dry and just scrape a little of the mold into the milk as it ripens.