In Google Map, go to your desired location and zoom in or out until the needed area is on screen. From the menu, select "Make Available Offline." A selection square is displayed and once you have everything you need inside the square, save the map.

You can then load the map any time, regardless of whether you have a cell signal, and the phone's GPS will show your location. I've not fiddled with offline navigation, but assume that's possible too. (Still a noob.)

Cheers,
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--Rick