Go to the 'Trips' tab (the rucksack icon on the bottom of the screen). Once your adventure is in 'Confirmed' status, a button will display to allow you to 'Download' your trip.
Make sure you are connected to a good wifi network, and tap this button. The download will take a short while as all the maps, at all the various zoom levels will be saved for offline use within the app.
Once the download is completed, your will see a confirmation message.
Navigate to the 'Maps' tab (the map icon in the bottom center of the screen) and your map will now be saved here for offline use. You can put your phone onto “flight mode” to test this.