In the morning of day 2, I stirred the starter, discarded half, and fed it 1/2 cup all-purpose flour and 6 tablespoons filtered water, mixing it up so there was no dry flour left.
The second day is when I got cocky and learned a valuable lesson. Using 100 percent all-purpose flour (which was the only kind I had on hand), was a mistake. It encouraged "bad bacteria" that made the starter look like it was an overachiever, getting bubbly and active and even doubling in size by the afternoon (which usually doesn't happen for multiple days). Warning: this is a false start that actually prolonged my starter growth in the long run.