what I do is I install the servo first, not worrying about the arm
then I connect the servo to the receiver and power on the transmitter and receiver, this zeros the servo.
then I ensure the thing to be controlled is where I want it (or as close as is possible) at neutral e.g. for steering the wheels would be straight
then I install the arm on the servo maybe making an adjustment to it to fit on the servo
all while the receiver and transmitter are on
then if further adjustment is needed I use the trim on the radio