Look first at the red dot in the middle of the screen. Try to look at the (whole) picture with a diffused view. When you see a pair of boxes of the same color, select both of them by left clicking on the matching box in the right row of colored boxes and also left clicking on the matching colored box in the left row of colored boxes. If your selection is correct, two green dots will appear.

Press the red dot at the center of the screen if you want to start a new task.