Dissolve the yeast in 1/4 cup warm water, about 105–110°F (40–43°C), and let it stand for about 10 minutes until dissolved and lightly foamy.

In a large nonreactive bowl, whisk together 1 cup all-purpose flour and 1 cup granulated sugar. Gradually stir in 1 cup room-temperature or slightly warm milk until smooth, then stir in the dissolved yeast mixture.

Transfer the mixture to a 1-gallon zip-top bag or a loosely covered nonmetal container, leaving room for gas as the starter ferments. This is Day 1.

On Days 2 through 5, gently squeeze the bag or stir the starter once each day to redistribute it. Keep it at room temperature and never seal a container airtight.

On Day 6, add 1 cup flour, 1 cup sugar, and 1 cup milk. Mix well, leaving extra room in the bag or container.

On Days 7 through 9, gently squeeze or stir the starter once daily. Watch for mild bubbling and a pleasantly yeasty or tangy aroma.

On Day 10, add another 1 cup flour, 1 cup sugar, and 1 cup milk, then mix thoroughly. Divide the mature starter into four portions of about 1 cup each.