Chef John's Sourdough Starter
The exact number of grams seen here doesn't really matter, as long as you're using exactly the same amount of flour and water, by weight.
I use gram weights for flour and water to ensure I'm using exactly the same amount for each. 700 grams of flour equals about 5 1/2 cups; 70 grams is a scant 1/2 cup. But I do recommend using a kitchen scale to measure grams.
For best results, use bottled water; chlorine can kill the yeast/bacteria. You can also use boiled, cooled water.
You can use different kinds of flour. I like half spelt and half bread flour, but this recipe will work with pretty much any combination, including all wheat flour.
Get the recipe for Chef John's Sourdough Bread.