Focusing smoothly along the string from the door attachment of the string to as close as you can see while keeping the crossing point of the two string images clear.
Hold on the inward position for 3 seconds then slowly slide your eyes outward.
Repeat this 10 times or for a minute. Take a break and then try again.
Turn your head slightly and try to focus up and down the string. One side may be much more difficult while the other side easier.
Your goal is to focus along the string as smoothly as you can without 'jumping' from point to point.