Make sure your servo is at it's electrical center. Place the double servo arm on the servo and make sure it is at 90 degrees to the servo. I assume you are using the threaded adjusters (have a small hole at one end for the wire to pass thru, clevis threads on other end). Make sure the clevises are screwed the same amount so each adjuster/clevis is the same length(put a locking nut on to tighten against the clevis). Thread your wires coming from the rudder through their guides and pass one end of each through a brass sleeve and then the tiny hole in the adjuster. Attach the adjuster and clevis to the servo arm in the desired holes(i usually start second one in ). clamp your rudder so it is straight with the vert stab. Pull each cable taut, just take out the slack, and bend it where it passes thru the adjuster hole. Double the end back through the sleeve, and then loop it over and back through the sleeve again, pull the slack out of these loops and then crimp the sleeve. This locks the cables in place so they don't slip.

If everything is done right, the cables will be almost tight, and both the same length, and the servo is still centered and the rudder is still straight. Loosen the locking nuts and turn the adjusters equally until the pull-pull has no play, but is not stretched and bending the rudder arm from tension. Retighten the locking nuts, power up and test the action. If the rudder stays centered, you're good to go, if not, back off one wire and tighten the other the same amount, until the rudder centers.