Prepare the Poolish
Mix 250 g of flour, 250 g of water, and 0.5 g of instant yeast in a bowl or container. Stir until no dry flour remains — the mixture will look like a thick batter rather than bread dough.
Cover the container loosely and leave the poolish at room temperature for approximately 12–16 hours. The exact time depends on the temperature of your kitchen.
A mature poolish is covered with bubbles and has increased noticeably in volume. Its surface may be slightly domed or just beginning to flatten in the center. The aroma should be pleasantly fermented, sweet, wheaty, and slightly creamy.
If the poolish has completely collapsed and developed a sharp alcoholic smell, it has fermented too long — it can still raise dough, but its flavor will not be as balanced.