When making an itinerary, don't worry about the start point. Start placing waypoints using the 'Point on map' function. Zoom in really close to be sure that you place the curser on the road, and in the correct lane. Use a pointy object like a stylus for precision. Your finger will be too thick.
Waypoints will be added to the list. Make sure that only the the last one is marked as a Destination (checkered flag). You do this by highlighting it and choosing this option.
You can save your itinerary, give it a name, and call it up whenever you want to use it.
Testing your itinerary can be tricky. This is because you must have a satellite link. But the unit wants to start the route from your present position. (I believe that you can do a simulation with Home, where you can set your start point where ever you want.)
If you don't like the routing, you may have to add more waypoints. Don't worry, you can move the points up and down on the list.
So do your experimenting by creating a route from you home position.
Hope this helps.