After many hours of research and struggle online to transfer easily a custom map from Google to my Garmin 660, I finally found an easy way to do so. For those interested, here is how I do it now since the Garmin Map Source software is just not great at all.
1- Create your own map by dragging your route through google map. (don't forget to save it) 2- You need to convert the link (of this specific map) into a GPX file and save it on your desktop or where ever you would like it to be. To do this conversion, I use this: http://www.cgtk.co.uk/navigation/gpx At the bottom of this page, you enter the name of your choice for this map (ex: beach trip) and then enter the link you got from Google and click "Convert to GPX" (in my case, it goes directly in the download folder--I'm using windows 7) 3- To transfer this GPX file to the GPS device connect it to the computer and ensure the device goes into mass storage mode. Windows should prompt you to open a folder to view files (if it doesn't then go to the Start Menu > My Computer and double click the Edge drive). From this Window Open the Garmin folder Open the GPX folder Paste the file into this folder (right click then paste) 4- Disconnect your GPS unit from the computer and turn it on. 5- Then, on your unit, go to the "tool" menu 6- Select the folder "my data" 7- then select "import data" 8- then select "route" (you should see your custom map with the name you entered in section 2-) Then click "Import" You should now see your custom map on your GPS.
You can re-access this map later by going in the "tool" menu then select "custom routes".