For this process, you need enough quantities of flour, salt, and water as well as a weighing scale and clean jars.
1. First day
Place an empty jar on a weighing scale, note the zero. Pour 100 grams of whole grain rye flour and 150 grams of warm water into a jar. Mix this thoroughly and afterward, keep it away from sunlight and let it rest for 24 hours.
2. Second Day
After letting the mixture rest for a day, you will start to see some fermentation activity. This will make the dough grow, but it soon subsides around the third or fourth day. To get permanent growth, keep at the process!
Place a second empty jar on the scale; make sure the scale reads 0 grams. Pour in only 75 grams of the mixture that has been resting for 24 hours; the rest should be thrown out.
Next, add 50 grams rye flour, 50 grams all-purpose flour, and 125 grams water — if it's cold, warm the water before using it. Mix well and place in a warm place for another 24 hours.
3. Third Day
Place your clean jar on the scale and tare. Scoop in 75 grams of the mixture that rested overnight and add 50 grams rye flour, 50 grams all-purpose flour, and 125 grams water. Stir thoroughly, then cover the jar and let it rest for 24 more hours.
4. Fourth Day
On the morning of the fourth day, you will see more signs of fermentation. All you have to do is follow the same process as you did the day before.
However, instead of a 24-hour resting period, let it rest for 12 hours. Discard down and refresh again with the same ratio of ingredients, and let rest overnight.
5. Fifth and Sixth Days
For days five and six, continue refreshing with the same ratio of ingredients as day four, twice daily, as fermentation progresses.
6. Day Seven And Onward
Place a clean jar on the scale and tare. Scoop in 50 grams of the mixture that fermented overnight. To this, add 50 grams rye flour, 50 grams all-purpose flour, and 100 grams water.
Stir well, cover, and let rest for 12 hours. Afterward, discard the jar contents down to 50g, add the same ratio of ingredients as you did earlier, and let it rest overnight.
7. The sourdough starter is ready for use when it consistently rises and falls
This indicates that it is just perfect for breadmaking. So bake away!
Note: As for what flour to use to feed your sourdough starter, you can use any based on preference.