To do what you want involves the use of Garmin's Basecamp and Basecamp is not intuitive.
Here is an outline of how I do it on a Windows machine:
In Google Maps one can plot out the points that one wants to visit.
And one may create a route from those points within google - unfortunately the route cannot be exported - only the points can be exported - the points are GPS coordinates and in no particular order when exported.
All the points can be exported to a KML/KMZ file and that KML/KMZ file can be imported into Basecamp.
Within Basecamp, select all the imported points, then right click and select "Create Route".
After the route is created, double click (or right click) the new route. In the next window click the "Optimize" button at the bottom of the screen - it will prompt you for the last destination and then optimize the route for you.
The Nuvi has to be plugged into the computer and you can then export the Basecamp file into the Nuvi GPS.
In the Nuvi the new route will be listed in the menu under Apps.
Simple, right?