Quick and dirty tutorial on downloading and searching coordinates on Google Maps. It's not totally intuitive, and it's changed recently.
This guide is written for Android, though iPhone I would hope is similar.
Open the Google Maps app. Tap the 3 horizontal lines in the upper left of the app (I've heard this symbol called the 'Hamburger')
Touch the 'Offline Areas' menu option
While this doesn't give you turn by turn navigation, it does give you the ability to search coordinates.
Other mapping software uses similar methods for offline use of maps. Personally, I have the paid version of Gaia GPS, which has USGS maps, as well as some open source hiking trail maps.