Windows users :

If you do not have Garmin Basecamp then download and install from here.

If you are concerned about malware, use VirusTotal to scan the download link to check. http://www.virustotal.com

The map download will be a zip file, unzip it to see a .gmap folder.

Copy the .gmap folder to C:\ProgramData\Garmin\Maps\

Start Basecamp and under Maps you should see GBOpen (GBOpenPeak for the sample). 
In Basecamp you can view the map, load and edit tracks and routes. 
If the Maps item is not visible use View>Toolbars and check Map Products

The sample map is 4MB, the whole of Britain is about 950MB. You will need this much free space 
on your Garmin device and I recommend that you load the map to a SD card rather than the 
main memory of the device.

To transfer to your Garmin device, connect it to your PC with a USB cable. 
Use Basecamp Maps>Install Maps and follow the instructions to load the map to your device.

Mac users :

If you do not have Garmin Basecamp then download and install from the Mac App Store

The map download will be a zip file, unzip it to see the .gmap folder.

Open the .gmap file with Garmin Map Manager

Start Basecamp and under Maps you should see GBOpen (GBOpenPeak for the sample). 
In Basecamp you can view the map, load and edit tracks and routes.

The sample map is 4MB, the whole of Britain is about 900MB. You will need this much free space 
on your Gamrin device and I recommend that you laod the map to a SD card rather than the main 
memory of the device.

To transfer to your Garmin device, connect it to your Mac with a USB cable. 
Use Maps>Install Maps and follow the instructions to load the map to your device.