Nmea out + (Blue wire(I think!)) of the garmin to the NMEA+ of the pilot
Black wire of the Garmin to the NMEA- of the pilot.
Garmin don't adopt a true NMEA output and use 0Volts (black wire) as the NMEA return (-) connection.
You then need to set the Garmine up to t's NMEA/NMEA output. if you then do a rout or a waypoint and make it active, the pilot shpuld then lock on to it and steer the boat accordingly. Try to get the boat roughly pointing in the right direction and close to track so as not to give it a major task to bring the boat round.