Make a loop in the wire at the end away from where the water rudder is.
Loosen the screws that hold the rudder horn to the rudder.
slip the loop under the nutplate on the side away from the rudder horn.
If one of the screws goes through the loop it is better.
Tighten the screws back down.
Here: read this, it has pictures. http://www.flyinglindy.homestead.com/skisandfloats.html
Click on the pages and they get big enough to read.