In a clean glass jar, mix 50g whole wheat or rye flour with 50ml unchlorinated water at room temperature. Stir vigorously to incorporate air. Cover loosely with a cloth or lid set ajar. Mark the level with a rubber band.
Leave at room temperature (ideally 21–24°C). After 24–48 hours you may see small bubbles — wild yeast is activating. The mixture may smell unpleasant at first — this is normal. Stir once daily but do not discard yet.
Discard all but 50g of starter. Add 50g fresh flour and 50ml water. Stir well. The discard reduces acidity and gives the healthy yeast more food. Repeat daily.
Continue discarding and feeding daily. You should see the starter double in size 4–8 hours after feeding. The smell will change to pleasantly sour and yeasty. It is ready to use when it reliably doubles and passes the float test.
Drop a small spoonful of starter into a glass of water. If it floats, it is ready to bake with. If it sinks, feed it again and wait. A mature starter can be kept in the fridge and fed weekly.