Oops, I missed a day. The good thing about starter is that a little bit of imprecision or neglect is unlikely to be fatal. This is how the starter looked today after a vigorous stirring and before I added any additional flour or water. By now, there are plenty of bubbles, and as expected, the starter is definitely smelling sour.
Sourdough Starter, Day 3 (or 4):
Add 2 tablespoons of flour and 2 tablespoons of water.
Stir vigorously. Cover and leave for another 24 hours.
Give the starter a vigorous stir a few times during the day.