If you have an SRF file, you can just drag it into your garmin vehicles file on your nuvi over a usb cable connection. If you want to make it 3 dimensional like the ones already on the unit follow the directions given in the file here: Vehicles for GARMIN nuvi
Here is one of the images I used.
I hope that posted right.
The actual file uses 72 images arranged like an old filmstrip, (boy, that really dates me!)
Hope this comes through allright!