Use an app that permanently downloads maps -- I like Mapy.cz and Organic Maps. And bring a waterproof phone case, an external battery, and a charging cable.
And also bring a waterproof paper map because it's lighter, easier to read, more resilient, and more fun.
OsmAnd is the pocket knife of cartography. It's by far the most flexible map I've use, although it's not as user-friendly as others. You can really choose what gets rendered and how.
And also bring a waterproof paper map because it's lighter, easier to read, more resilient, and more fun.