Prepare starter at least 2 days before you plan to use it. Mix flour and water until smooth in a 1 quart bowl or glass jar; add sugar and stir until dissolved. Cover loosely and let stand in warm (about 80 F), draft free place until fermented (mixture will be bubbly and smell sour); stir from time to time during fermentation.For the bread, sift flour with salt and sugar and set aside. Pour warm water in a warm large bowl, sprinkle in yeast, and stir to dissolve.Mix in starter, then flour (a little at a time), working last bit in with your hands.Knead on a lightly floured board until elastic (8 to 10 minutes), shape into a ball, place in a greased bowl, and turn to grease all over. Cover with cloth and let rise in a warm, draft free place until doubled in bulk, about 1 hour.Punch down, knead lightly 1 to 2 minutes, and divide in half. Shape into 2 round or long tapering loaves and arrange 3 “ apart on a lightly floured baking sheet. Cover and let rise until nearly double in bulk.Toward the end of rising, preheat oven to 400 F. Brush tops of loaves with cold water and, if you like, make diagonal 1/4” deep slashes the length of the loaf with a sharp knife. Bake (with a shallow pan half full of hot water on shelf below loaves) 35 to 40 minutes or until well browned and hollow sounding when tapped. Cool on wire racks before cutting.