Like Dash8Tech says, it's very simple. The Piper Aztecs I fly are fitted with the system, it's just a simple antennae, let the system orientate itself before departure (but it's not like INS, you don't have to sit there for ages stationery, it's a few seconds), then program all the waypoints which are beacons (by entering the three letter ICAO codes) which are in it's memory, and there's a 'moving map' screen that clips onto the yoke and visually draws the airways, beacons, suitable fields en route etc. It takes about five minutes to enter all the waypoints. We get a small cartridge once a month or so and that has all the updated info. After departure you couple the autopilot to the nav mode and the GPS navigates the plane from beacon to beacon, and we retune the nav radios as we go, to crosscheck the accuracy of the system, which so far has been flawless.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz