modifying old tx for aileron rudder coupling
Since your profile doesn't show where you are, I will assume somewhere in the US. In North America, it is illegal to make any modifications to the transmitter without the appropriate licenses. If you want to do mixing with a non-computer radio, there are 2 options I know of:
1). There are "receiver add-ons" which do the mixing electronically. Just plug the mixer into both ports on the receiver, and the servos into the mixer.
2). Build a mechanical mixer (like it used to be done). The aileron servo is mounted in a sliding tray, which is moved by the rudder servo.