I tape the servo lead to the body of the servo to keep it positioned roughly between the screws, then use the wire w/ extension attached and a stick to guide the servo into position. I made a long allen wrench extension using a dowel and a ball socket dowel wrench which was magnetized.

Put the servo arm on the servo and correctly align it prior to installing in the wing, raise the servo arm through the slot in the wing sking prior to screwing down the servo. Screw the servo down using the socket head servo screws that are available. Not to hard after you do it once.

Alternatly cut a hatch in the wing and cover the opening with tape and the cutout from the skin when the servo is in.