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1
Combine 100 g rye flour and 100 ml water in a clean glass jar; stir until homogeneous and leave uncovered at room temperature (68–72 °F / 20–22 °C).
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2
Observe for 24–48 hours; wild yeast and Lactobacillus will colonize the mixture, producing bubbles and a slightly sour aroma.
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3
Discard half the starter (50 g) and feed with 50 g flour and 50 ml water; stir, cover loosely, and rest 24 hours.
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4
Repeat step 3 daily for 5–7 days until the starter doubles predictably within 4–8 hours of feeding and shows a pleasant, tangy smell with a stable ratio of rising/falling cycles.
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5
Once mature and doubling reliably, refrigerate between feedings; feed once weekly or before use (remove from cold storage 12 hours ahead, feed, allow to peak at room temperature, then refrigerate).
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6
For baking, use starter at its peak (fully risen, visibly bubbly, doubled in volume from feed); discard half, feed with flour and water equal to the remaining mass, and let rise 4–6 hours before incorporating into dough.