This tutorial demonstrates how you can easily create a very active sourdough starter culture using only bread flour, filtered water, and organic apple. Start baking your own sourdough at home in no time! See the full written instructions at:
https://homesteadandchill.com/make-yo...


 
Step 1: Mix 500g white bread flour, 1 large or 2 small grated organic apples (use the skin, avoid the core), + 360 ml of room temp filtered water. Add to an air-tight container, large enough to allow for at least doubling in size. Cover and let ferment for 3 days. A spot that is about 70-75 degrees is ideal. Mark the side of your container so you can monitor its growth.
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Step 2: After 3 days, the mixture should be a bit bubbly, risen, smell sweet & tangy, maybe darken in color. It’s time to do the first discard and feed! Stir the mixture, discard half, and add in another 250 g flour + 170 ml of tepid water. Mix thoroughly. You can do this either in a separate bowl, and put it back into a now-clean ferment vessel, or like we do, mix it in place but clean up the sides a bit so you can still monitor the rise and fall. Re-mark if needed. Let sit for 2 days.
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Step 3: After 2 more days. There should be a lot of activity and small bubbles now. If not, allow to sit another day or two. Repeat discard and feed steps from Step 2, but this time making it more wet, like a "wet, sloppy dough". Let sit again for 24 hours.
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If your starter is bubbly and active after that last 24 hours, congrats, you did it! You now have a happy sourdough starter that is ready for baking! If you bake frequently or have the free time, you can store the starter on your countertop but it must be fed DAILY (even 2x a day).   
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Instead, we store ours in the fridge, pull it out 1-2 days before baking, let it warm up, and feed it at least twice prior (e.g. take out Thu night, feed Friday morning & early afternoon, make dough Fri evening). We don't weigh ingredients when we feed. We use a smaller container than shown in the tutorial, discard to leave about 1/2 to 3/4 cup of active starter, then add 1 scant cup flour and 1/2 cup tepid water. It’s easier than using the scale, and works fine! For an extra active starter, try feeding part white and part rye or whole wheat. Don't be alarmed if your starter gets a thin layer of dark liquid on top. That is just the sign of an over-active, hungry starter. Mix it in and feed as usual.