1 If there isn't a folder on the SD card called 'Garmin', create one
2 Into this folder, copy the .img file that you downloaded (*.img).
3 Rename it to gmapsupp.img if it's not already called that
Back to your Garmin
Rename or delete original *.img at your garmin internal memory. I like rename it as I can roll it back when I need it again.
and you can go from there,
Good luck!