Stand with your arms out in front of you, one directly above the other and approximately thirty centimetres apart.
Stick your thumbs out so that they are pointing towards opposite walls.
Start by looking straight ahead, in between both thumbs.
Keep your head still and move your eyes to look up at the top thumb.
Focus.
Then lift up your head so that you are looking directly at the same thumb.
Keep your head still and move your eyes to focus on the bottom thumb.
Focus.
Move your head down to look directly at this thumb.
Continue moving between the two targets in this fashion.