Gather tools: digital scale, glass jar (3/4 L), rubber spatula.
Day 1: Weigh 60g whole wheat flour + 60g warm water (filtered if possible) into jar. Mix until smooth (pancake batter consistency). Cover loosely, rest 24h in warm spot (70-75°F/21-24°C).
Day 2: Look for bubbles. No feeding, rest 24h.
Days 3-7: Discard half (~60-116g daily), add 60g unbleached all-purpose flour + 60g water. Mix, cover, rest 24h warm. Mark jar to track doubling.