Take a gallon jar or crock and put in:

2 c. flour
3 tbsp. sugar
2 c. lukewarm water
1 tsp. salt
1/2 tsp. dry yeast

Stir until mixture is a smooth paste. Put on lid and set in a warm place to sour. Stir it several times a day. In 2 or 3 days your sour dough will be ready.