I read the info at this site, talked with one of the suppliers he
recommended in Atlanta, met him at a shopping mall, bought and installed
the springs all in one day. All I needed to purchase was the pair of
winding rods, which I needed anyway since I had just bought three
sectional doors for my new shop that I was about to install. The
repair of the first door, and the installation of the next three
couldn't have gone more smoothly. A helper might have been beneficial,
but not necessary.
But, if you insist, at least go here and study, study, study before you
start:

http://www.truetex.com/garage.htm