Practice binocular vision
Hold one end of the Brock String to the tip of your nose and tighten the string. Start with smaller distances between the balls. Focus on the first sphere with both eyes.
The aim is to see two entering and two exiting strings exactly in the center of the hole of the focused ball. (See illustration).
Now gradually focus on the balls behind. The angle of the string to the tip of the nose and the distance between the balls can be varied to increase the level of difficulty.