In a large non-metallic container, such as a 2-quart glass jar, combine 3 1/2 cups of all-purpose flour and 2 1/4 teaspoons (or one package) of regular or quick active dry yeast.

Gradually add 2 cups of very warm water to the flour mixture, beating until the mixture becomes smooth and well-combined. The water should be warm to the touch but not hot.

Place a plate underneath the container to catch any potential overflow, as the starter will expand during the fermentation process.

Cover the container with cheesecloth or another breathable material to allow air circulation while preventing contaminants from entering the starter.

Store the covered container in a warm, dry location for 4 days, or until the starter develops a distinctly sour aroma, indicating that the fermentation process is complete.

Once the starter has fermented, transfer it to a plastic airtight container and store it in the refrigerator until you're ready to use it in your sourdough recipes.