Day one: Get a 1 liter jar. Pour a cup of spring water and add a cup of rye flour. Mix it well together, cover it with a cloth, and let the mixture stand for about a day (12 to 24 hours) on a counter, at room temperature.
Day two: Pour out half the mixture, add another cup of water and another cup of flour. Mix it, cover it, and let it sit on the counter for another day.
Day three: By this point you should see some bubbles forming at the top of the mixture. And once again, pour out half the starter mixture, add a cup of water and a cup of flour. Cover the jar, and let it sit on the counter for another day.
Day four: Your starter should be good to use and you can begin using it to make a simple bread without commercial yeast.