A 100% sourdough starter is made entirely from flour and water, with no commercial yeast. To create one, mix equal parts (by weight) of flour and water in a jar or container. Whole grain flours like rye or whole wheat are great for beginning a starter due to their rich nutrient content. Maintain the starter by regularly discarding part of it and feeding it with fresh flour and water until it's active and bubbly. This process can take a few days to a week or more, depending on various factors like temperature and the type of flour used.