A sourdough starter is a culture of wild yeast and lactic acid bacteria. Making one is simple: mix flour and water, feed it daily, and wait. Day 1: combine 50 g whole rye flour and 50 g water (room temperature) in a clean jar. Stir well, cover loosely, leave at 24–26°C.
Days 2–6: discard all but 50 g, add 50 g rye flour and 50 g water. Stir, cover, wait. By day 3 you'll see bubbles and smell something sour, possibly unpleasant. That's normal — leuconostoc bacteria dominate first, then lactobacillus takes over. By day 5–6, the starter should double in volume within 4–6 hours of feeding. If it doesn't, keep going — some starters take 10 days.
Day 7+: switch to your baking flour if you like (I use 50/50 bread flour and rye). Feed once or twice daily at room temp, or once a week in the fridge. The starter is ready to bake with when it reliably doubles in 4–6 hours after feeding. The float test (drop a spoonful in water — if it floats, it's ready) works, but watching it double is more reliable.