After you have purchased the HD Hillshade for Garmin we recommend that you download it to your computer. After extracting the ZIP file you will have a single folder called "GPSmap.is Hillshade for Garmin v1" with a single file inside called "gpsmapis-dem.img". The file is 126 MB in size.
Connect your Garmin device to your computer via USB cable. The Garmin device should then be visible in Windows File Explorer as a new drive.
NOTE: If the device is not visible after 30 seconds or so, you may need to change the MTP Setting. See here.
To install the IMG map, look for a folder in the device called "Map". You should already have our Garmin map installed in that folder. Copy the "gpsmapis-dem.img" file into that folder. Then unplug and restart your garmin device.
After your device is up, go to Settings > Map & Vehicle > myMaps and you should see this view here:
Make sure you check the "GPSmap.is DEM v1" entry in the list. That's it, now you can enjoy your map with HD Hillshade.