Initially the trimming is 0 for all servos. In this case the Status LED will be off when entering Setup Menu Point J. At first, attach the servo horns to the servos in this position. So you will make sure to get roughly equal throws to both directions. Mount the servo horns so that they form a 90 degrees angle to the linkage rod as close as possible. Then in the next step you can fine trim every single servo‘s center position, as usually mounting the servo horns at exact 90 degrees will not work out perfectly, depending on the servo gear trains and the servo horns.
By tapping the rudder stick you can select one servo after another. Every color of the Status-LED is corresponding to a specific servo channel that is indicating its selection by a short up and down move. Use the aileron stick to change the servo trimming/adjust the center position. You can switch back and forth between the servos with rudder as often as you need.