Step 1 Sprinkle the yeast into warm water, stir and let stand for 5 to 10 minutes.
Step 2 Mix the bread flour, all-purpose flour, whole-wheat flour and rye flour in a bowl. Measure one-half teaspoon of the yeasted water into the flour mixture. (Throw the rest away; the point of this step is not to proof the yeast but to measure 1/384 teaspoon yeast.)
Step 3Add the three-fourths cup water, using ice water in the summer and warm water in the winter. Thoroughly mix the biga; it will be stiff, but it has a very long fermentation and will soften considerably. Knead in a tablespoon or two of water if you absolutely must.
Step 4Cover tightly with plastic wrap and let the biga ferment for 24 hours in a cool spot in the summer or a warm one in the winter. Don't be alarmed if it does nothing for at least 10 hours; this is correct. The biga is ready when it doubles or triples in volume and smells aromatic.