Put your container on the weighing scale and zero it. If you have no weighing scale, use measuring cups. The idea is to feed your starter with equal amount of water and all-purpose flour.
Weigh your starter. Although I usually set aside 100 gm each time, it always fall short. And that’s okay.
Zero your weighing scale again and warm a glass of filtered water before mixing with the starter. I use the microwave for 30 seconds. The water should be warm enough to give your baby starter a bath, and not scald it. You don’t want to kill your bacteria and natural yeast!
Pour an equal amount of warm filtered water into your plastic container.
Mix the water and the starter with plastic mixer.
Zero the weighing scale with the water and starter on it. Then measure an equal amount of unbleached, unbromated all-purpose flour.
Mix the flour, starter, and water.
Pour 100 gm of the mixture into an empty jar. This will be your reserved starter for future use. Don’t forget this step otherwise, you won’t have something to use the next time you want to bake a sourdough bread.
Technically, you can make your own starter but why bother training a young starter when you have a mature one with predictable ripening time. My 400-year-old starter bubbles at 12-18 hours from feeding.
Store your reserved starter in the fridge after writing the date of last feeding. My starter is resilient and can go without feeding for even a month or more.
Mark the level of the starter on the container with an erasable pen and write the time.
I usually maintain the temperature by putting the small container in a large plastic cover like a disposable cake cover. My incubating temperature is between 70-80F. I expect this starter to bubble and rise above the level in 12 hours. Then we can start baking our sourdough bread.