Mix the flour, sugar, and yeast together in a clean and sterile container (use only glass, glazed ceramic or crockery to hold your starter. No metal or plastic) that can hold two quarts. Gradually stir in the water and mix until it forms a thick paste (do not worry about any lumps, as they will disappear).
Cover the container with a dish cloth and let it sit in a warm (70 to 80 degrees F.), draft-free place. NOTE: Temperatures hotter than 100 degrees F. or so will kill the yeast. The dish cloth will let wild yeasts pass through into the batter. The mixture should bubble as it ferments (this will foam up quite a bit). Sometimes I place the container in my sink (if sourdough spills out onto your counter, it is hard to clean off once it has dried).
Let it sit out (at room temperature) for 2 to 5 days, stirring it once a day. The starter is ready when it develops a pleasant sour smell and looks bubbly.
Once your starter starts bubbling, then start feeding it daily with flour and water according to the directions below. Then stir it, cover loosely with plastic wrap (allow a little breathing space), and store it on your counter top or in the refrigerator (your choice).
* Adding a little sugar will help jump start the yeast process, as yeast feeds on sugar for its energy. Yeast rises by feeding on the sugars in flour, and expelling carbon dioxide in the process. That is why using just a little sugar can help boost this process. Do not overdo the sugar.
** If the water you use contains chlorine, use distilled water, bottled water, or tap water that you have allowed to set out for 24 hours when you make your starter. Chlorine can stop the development of yeast.
Easy Sourdough Starter Recipe: https://whatscookingamerica.net/Bread/SourdoughStarter.htm