Here’s how it works:
Login to the eMusic app, go into My Music and then into Playlists.
Click on a playlist and then press the toggle to make it available offline.
Easy, ain’t it?
Mind you, you have to be online for the tracks to download onto your device. You can undo that action at any point by just pressing the toggle again, keeping the option of easily returning it to an offline playlist OR remove the album from device, individually or as a whole through the playlist’s More Options menu.
If you wish to, you can add more tracks to the (offline) playlist and they will automatically be downloaded and included with the rest of the songs.