Combine the water and flour in a mixing bowl, stir well, then knead until smooth on a non-floured wooden board, roughly 3-5 minutes.
Return to bowl and cover tightly with a towel or cheesecloth.
Leave at room temperature for 12/15 hours, then add another 1/2 cup flour and 1/4 cup lukewarm water and mix thoroughly (called feeding the starter).
Cover and rest again for another 12-15 hours, repeating another time for 24 hours. If it's risen and bubbly repeat the process a couple more times—or longer for a richer loaf—then taper down to a 12 then 4-8 hour window, tightly covering so no air gets in.
When it's ready it should be bubbly with a tangy smell.
Store in a Weck canning jar with a glass lid. You can also use a wide-mouth mason jar.