I wrote about this on my hiking blog, here. It’s definitely a useful navigating feature for areas without cellular data connection! If you still can’t get it to work, here’s an animated video of how to navigate offline using Google maps on android device. HT to Gusrix for the video.
For first time iMessage and Facetime users, your device must send an international message to Apple as part of the registration process. That's going to appear on your postpaid bill or be deducted from your prepaid credit.