1. Open Amazon Music by clicking on the three white horizontal lines at the bottom of your screen and selecting the app. Sign in to your account.
2. Choose a playlist from your library, and open it.
3. Download the playlist by pressing the 4th icon from the left, under the playlist title.
4. Once your playlist is downloaded, test it out by playing it with your WiFi off. If your music plays while your WiFi is off, you're all set!
(Tip: You can turn your WiFi off by swiping down from the top of your screen until you see a volume slider, and selecting WiFi to turn it off. Then, you can swipe that screen away by putting your finger at the bottom of your screen and swiping upward. You can also turn it off by going back to your home screen and clicking the blue question mark to open the Help App > Open Android Settings > Network > WiFi > then toggling WiFi off at the top right.)