The simplest way for most tiller rudders is to make a simple yoke from the servo arm.
Install the servo on its side under the rear deck so that the arm is facing forwards and the arm is pointing upwards. Make a simple yoke, a long u shaped piece of stiff wire so that the width of the hoop fits over the tiller. It needs to extend above the tiller so it can swing freely. Attach the bottom of the wire hoop to the servo arm. As the arm swings each way the hoop slides over the tiller swinging it from side to side.
Sounds complicated, but it's dead simple.
Chas
