I need to find out the best way.I know in principle how to do it and done it in the past.You have to put the spring under tension wind the string on the opposite way to the tension,so when you pull the string it tensions up the spring and the recoil pull the string back on the spool.Be careful when to place the spool back as often because its under tendion it will uncoil again,which is annoying.I was hoping to find a diagram of it,which would be helpful,or even a video.
Hope this helps it is an art.
Phil Passfield, June 2008