Purchase a sourdough starter and follow the directions to activate it.
1 cup sourdough starter
Alternatively, you can see if you have a friend who can gift you some starter. You only need about ½ cup to get started.
Weigh your starter. Add an equal amount of flour and water to that measurement. Stir, cover loosely, and allow to sit at room temperature until bubbly. The starter will have a distinct yeasty aroma.
1 cup flour, 1 cup water
Once your starter is active and bubbly, you can use it in recipes.
After you use your starter, you can discard half or use it in recipes that call for "discard".
Use the starter continually in recipes, repeating the process of feeding it, at least once a week.
A light gray tinge or liquid on the top of the starter is normal. Just stir it in. However, if your starter starts to smell like nail polish remover or develops orange or pink coloration or mold, it has gone bad and must be thrown out.To use sourdough starter in a recipe that doesn't already call for starter: This will take some trial and error to make adjustments to your favorite recipe, but here's a rule of thumb. Substitute 8 ounces of starter for each packet of yeast the recipe calls for, then omit 4 ounces of flour from the recipe as well as 4 ounces of liquid, such as milk or water. Note this will only work in recipes that already have an adequate amount of liquid and flour.