Work out how long you need the ramp to be. Estimate the highest potential difference in height between the floating dock and the jetty. This will occur when the water level is at its lowest. At this level, the angle of the ramp should be 45 degrees, meaning that the gradient is 1:1. Work out the length of the ramp using Pythagoras' theorem: Length of the ramp squared = 2 x maximum height difference between jetty and floating dock squared. So if the maximum distance is 6 feet, the length of the ramp will be around 8 feet 6 inches. Cut this length of wood.
Attach the ramp rollers to the end of the plank that will be on the floating dock. These are a form of a bracket, which will allow the plank to move as the dock rises up and down.
Hammer two fence staples into the end of the plank that will stay on land. Because of the changing angle of the ramp, it cannot be directly attached to the jetty. Tie rope or sturdy string through the fence staples and tie them onto mooring cleats if they are available, or to fence staples that you have hammered into the jetty. Your ramp will now be secure but will respond to changing water levels.
Sand the ramp down once you have fitted it, to prevent splinters. You may also wish to varnish it.