In a quart sized jar, add the cup of flour. Add the cup of warm water and mix into a paste. Screw on the lid tightly and set aside on the counter for 24-48 hours.
After about 24 hours you should see some bubbling activity. If it has bubbled significantly, it’s time to feed it! Discard all but ½ cup of the starter and add ½ cup whole wheat flour and ½ cup water and mix. Screw the lid on and set aside again for another 24 hours. If the starter is bubbling up in less time, switch to feeding twice a day instead of once a day.
Do this for 6 days and when you reach the 6th day, instead of discarding half of the starter, remove half and set aside to use in your sourdough bread.
If you don't have whole wheat flour, you can use all purpose flour. It may not get as bubbly as it would with whole wheat flour, but it will work! If you're using your starter less often, you can keep it in the fridge instead of on the counter top. Having it in the fridge will slow the fermentation process so you will only have to feed it once every week or so. If you forget about it, it can be reactivated even after a couple months of neglect. The longer it sits in the fridge, the more dark liquid may form on top of it. This is called hooch and can be mixed back in or poured out before you feed it. Be sure to let the starter come to room temp before feeding and then allow it to sit on the counter for 12 hours or so until you return it to the fridge.If you would like to increase the amount of starter you have first make sure your jar is big enough to allow it to double in size and then mix equal parts starter, flour, and water.