Hold a bright toy about thirty centimetres from your baby's face and wait until their eyes settle on it.
Move it slowly to one side, then back across to the other. Slower than feels natural — roughly the speed of a second hand.
Stop the moment they look away. Four or five passes is a full session at this age; a baby who breaks gaze is finished, not distracted.
Smooth visual tracking is a motor skill of the eyes, and it has to be trained before a baby can follow a rolling ball or, much later, a line of text. Moving too fast breaks the pursuit into jumps and trains nothing.