Stand with one foot in front of the other, heel to toe.
Hold your arm out in front of you, with your thumb pointing towards the ceiling.
Focus on your thumb.
Move your thumb to the left as you turn your head to the right.
Then smoothly rotate your head to the left whilst you move your thumb to the right.
Your gaze should still be focused on your thumb.
Gradually increase how long you can tolerate this for, and then increase your speed.