After you download the app, your music library will be synced from iTunes, and any additional purchases from the iTunes store will automatically be added to Listen.
The application is controlled with swipe gestures. You can swipe left or right to switch to the previous or the next song respectively. Swiping down brings up the menu where you can select an album playlist or radio, and by swiping up, you can add the current song to favorites, set Listen to shuffle mode, repeat or navigate back the the music selection menu.