As Scotty said, you are transferring waypoints from the mapping program(google maps) to the garmin. The garmin software then computes the route from waypoint to waypoint. The key is to lay out the waypoints in such a way that it forces the garmin software to compute the route you want. A few years ago, when we went on our route 66 ride, I think we had over 1000 waypoints to keep us on 66. Otherwise, the garmin would throw us on I-40 or something else.
I never figured out exactly how to use google maps to set up the waypoints necessary. I also found Mapsource to be pretty complex. I finally settled on the Harley Davidson Ride planner. It does the same thing from the standpoint of transferring waypoints, not the route itself, but I found it the easiest to add waypoints in the order I wanted.