A simpler way is offered by Qlandkarte GT. It works like MapSource.
From the Main panel select the Map entry. Then click on entry Select Sub Map, and select with the mouse the area you’re interested in. Needed tiles will be automatically selected and shown. In the Selected Map panel you’ll have info about number of tiles selected and size. Select that entry and click on button Export Map. Choose an output path and confirm with Export. When it finishes you can manually transfer the output file to the Garmin device via USB.
