First of all visit the G Map Tool website especially the US OSM Topo Routable page.
Then pick the area you like and download the correct map package.
Link your Garmin GPS device to your computer and make sure it’s ON. Give it a minute or two to load and register it with your computer.
Load the application for Garmin Map Install, your GPS device should be shown in the top window, hit continue.
Garmin Map Install can be used for maps installation or removals.
Map Install will show you what maps you have already installed (if any), and how much space you have got on your device (at bottom). Click “send maps” or continue and wait. It can take a while for maps to be loaded onto your device.
Now confirms that the maps are being loaded, go to your map settings and look around.
Installing free Maps to your Computer (for WINDOWS)
Map source software is only available for windows and can be downloaded from the Garmin Map source website for free.
Connect your Garmin device to your computer with a USB cable and start Map source.
Click on the tools menu, and then click the map option.
Select the regions of the map you want to load.
Click the transfer menu and then click the send to device option.
Make sure that the device appears in the device field below the find device button and then click send.
Once Map source has finished loading the maps to the device, you can disconnect GPS and the maps will be ready for use on the device.
Installing free Maps to your Computer (for MAC)
To install the maps, you will need three free programs.
The first one is a 7z extractor. The map download is in 7z format, which is a compressed fancy zip file.