Start with 25 grams of water and 25 grams of flour. Use room temperature or cold water, but not warm water.
Discard half a mixture, so you end up with 25 grams of “discard” (which will not really look like discard at that point, only flour and water mixed). Repeat the process from day 1: feed it with 25 grams of water and 25 grams of flour.
Discard half (that means the remaining starter should weigh around 38g) and add the same amount of water and flour: so 38 grams of water and 38 grams of flour.
Discard half (that means the remaining starter should weigh around 57g) and add the same amount of water and flour: so 57 grams of water and 57 grams of flour.
Fetch a clean jar and pour 30 grams of starter in, discard the rest. Add 30 grams of water and 30 grams of flour.
Discard half (that means the remaining starter should weigh around 45g) and add the same amount of water and flour: so 45 grams of water and 45 grams of flour.